X. ^A

GENESIS, HEREDITY, AND VARIATION

CONCLUDED.

§ 97a. Since the foregoing four chapters were written, thirty-four years ago, the topics with which they deal have been widely discussed and many views propounded. Ancient hypotheses have been abandoned, and other hypotheses, referring tacitly or avowedly to the cell-doctrine, have been set forth. Before proceeding it will be well to describe the chief among these.

Most if not all of them proceed on the assumption, shown in § 66 to be needful, that the structural characters of organisms are determined by the special natures of units which are intermediate between the chemical units and the morphological units--between the invisible molecules of proteid-substances and the visible tissue-components called cells.

Four years after the first edition of this volume was published, appeared Mr. Darwin's work, The Variation of Animals and Plants under Domestication; and in this he set forth his doctrine of Pangenesis. Referring to the doctrine of physiological units which the preceding chapters work out, he at first expressed a doubt whether his own was or was not the same, but finally concluded that it was different. He was right in so concluding. Throughout my argument the implication everywhere is that the physiological units are all of one kind; whereas Mr. Darwin regards his component units, or "gemmules," as being of innumerable unlike kinds. He supposes that every cell of every tissue gives off gemmules special to itself, and capable of developing into similar cells. We may here, in passing, note that this view implies a fundamental distinction between unicellular organisms and the component cells of multicellular organisms, which are otherwise homologous with them. For while in their essential structures, their essential internal changes, and their essential processes of division, the Protozoa and the component units of the Metazoa are alike, the doctrine of Pangenesis implies that though the units when separate do not give off invisible gemmules the grouped units do.

Much more recently have been enunciated the hypotheses of Prof. Weismann, differing from the foregoing hypotheses in two respects. In the first place it is assumed that the fragment of matter out of which each organism arises consists of two portions--one of them, the germ-plasm, reserved within the generative organ of the incipient individual, representing in its components the structure of the species, and gives origin to the germs of future individuals; and the other of them, similarly representative of the specific structure, giving origin to the rest of the body, or soma, but contains in its components none of those latent powers possessed by those of the germ-plasm. In the second place the germ-plasm, in common with the soma-plasm, consists of multitudinous kinds of units portioned out to originate the various organs. Of these there are groups, sub-groups, and sub-sub-groups. The largest of them, called "idants," are supposed each to contain a number of "ids"; within each id there are numerous "determinants"; and each determinant is made up of many "biophors"--the smallest elements possessing vitality. Passing over details, the essential assumption is that there exists a separate determinant for each part of the organism capable of independent variation; and Prof. Weismann infers that while there may be but one for the blood and but one for a considerable area of skin (as a stripe of the zebra) there must be a determinant for each scale on a butterfly's wing: the number on the four wings being over two hundred thousand. And then each cluster of biophors composing a determinant has to find its way to the place where there is to be formed the part it represents.

Here it is needless to specify the modifications of these hypotheses espoused by various biologists--all of them assuming that the structural traits of each species are expressed in certain units intermediate between morphological units and chemical units.

§ 97b. A true theory of heredity must be one which recognizes the relevant phenomena displayed by all classes of organism. We cannot assume two kinds of heredity, one for plants and another for animals. Hence a theory of heredity may be first tested by observing whether it is equally applicable to both kingdoms of living things. Genesis, heredity, and variation, as seen in plants, are simpler and more accessible than as seen in animals. Let us then note what these imply.

Already in § 77 I have illustrated the power which some plants possess of developing new individuals from mere fragments of leaves and even from detached scales. Striking as are the facts there instanced, they are scarcely more significant than some which are familiar. The formation of cauline buds, presently growing into shoots, shows us a kind of inheritance which a true theory must explain. As described by Kerner, such buds arise in Pimpernel, Toad-flax, etc., below the seed-leaves, even while yet there are no axils in which buds usually grow; and in many plants they arise from intermediate places on the stem: that is, without definite relations to pre-existing structures. How fortuitous is their origin is shown when a branch is induced to bud by keeping it wrapped round with a wet cloth. Even still better proved is the absence of any relation between cauline buds and normal germs by the frequent growth of them out of "callus"--the tissue which spreads over wounds and the cut ends of branches. It is not easy to reconcile these facts with Mr. Darwin's hypothesis of gemmules. We have to assume that where a cauline bud emerges there are present in due proportions gemmules of all the parts which will presently arise from it--leaves, stipules, bracts, petals, stamens, anthers, etc. We have to assume this though, at the time the bud originates, sundry of these organs, as the parts of flowers, do not exist on the plant or tree. And we have to assume that the gemmules of such parts are duly provided in a portion of adventitious callus, far away from the normal places of fructification. Moreover, the resulting shoot may or may not produce all the parts which the gemmules represent; and when, perhaps after years, flowers are produced on its side shoots, there must exist at each point the needful proportion of the required gemmules; though there have been no cells continually giving them off.

Still less does the hypothesis of Prof. Weismann harmonize with the evidence as plants display it. Plant-embryogeny yields no sign of separation between germ-plasm and soma-plasm; and, indeed, the absence of such separation is admitted. After instancing cases among certain of the lower animals, in which no differentiation of the two arises in the first generation resulting from a fertilized ovum, Prof. Weismann continues:--

"The same is true as regards the higher plants, in which the first shoot arising from the seed never contains germ-cells, or even cells which subsequently become differentiated into germ cells. In all these last-mentioned cases the germ-cells are not present in the first person arising by embryogeny as special cells, but are only formed in much later cell-generations from the offspring of certain cells of which this first person was composed." (Germ-Plasm, p. 185.)

How this admission consists with the general theory it is difficult to understand. The units of the soma-plasm are here recognized as having the same generative powers as the units of the germ-plasm. In so far as one organic kingdom and a considerable part of the other are concerned the doctrine is relinquished. Relinquishment is, indeed, necessitated even by the ordinary facts, and still more by the facts just instanced. Defence of it involves the assertion that where buds arise, normal or cauline, there exist in due proportion the various ids with their contained determinants--that these are diffused throughout the growing part of the soma; and this implies that the somatic tissue does not differ in generative power from the germ-plasm.

The hypothesis of physiological units, then, remains outstanding. For cauline buds imply that throughout the plant-tissue, where not unduly differentiated, the local physiological units have a power of arranging themselves into the structure of the species.

But this hypothesis, too, as it now stands, is inadequate. Under the form thus far given to it, it fails to explain some accompanying facts. For if the branch just instanced as producing a cauline bud be cut off and its end stuck in the ground, or if it be bent down and a portion of it covered with earth, there will grow from it rootlets and presently roots. The same portion of tissue which otherwise would have produced a shoot with all its appendages, constituting an individual, now produces only a special part of an individual.

§ 97c. Certain kindred facts of animal development may now be considered. Similar insufficiencies are disclosed.

The often-cited reproduction of a crab's lost claw or a lizard's tail, Mr. Darwin thought explicable by his hypothesis of diffused gemmules, representing all organs or their component cells. But though, after simple amputation, regrowth of the proximate part of the tail is conceivable as hence resulting, it is not easy to understand how the remoter part, the components of which are now absent from the organism, can arise afresh from gemmules no longer originated in due proportion. Prof. Weismann's hypothesis, again, implies that there must exist at the place of separation, a ready-provided supply of determinants, previously latent, able to reproduce the missing tail in all its details--nay, even to do this several times over: a strong supposition! The hypothesis of physiological units, as set forth in preceding chapters, appears less incompetent: reproduction of the lost part would seem to be a normal result of the proclivity towards the form of the entire organism. But now what are we to say when, instead of being cut off transversely, the tail is divided longitudinally and each half becomes a complete tail? What are we to say when, if these two tails are similarly dealt with, the halves again complete themselves; and so until as many as sixteen tails have been formed? Here the hypothesis of physiological units appears to fail utterly; for the tendency it implies is to complete the specific form, by reproducing a single tail only.

Various annulose animals display anomalies of development difficult to explain on any hypothesis. We have creatures like the fresh-water Nais which, though it has advanced structures, including a vascular system, branchiæ, and a nervous cord ending with cephalic ganglia, nevertheless shows us an ability like that of the Hydra to reproduce the whole from a small part: nearly forty pieces into which a Nais was cut having severally grown into complete animals. Again we have, in the order Polychætæ, types like Myrianida, in which by longitudinal budding a string of individuals, sometimes numbering even thirty, severally develop certain segments into heads, while increasing their segments in number. In yet other types there occurs not longitudinal gemmation only, but lateral gemmation: a segment will send out sideways a bud which presently becomes a complete worm. Once more, Syllis ramosa is a species in which the individual worms growing from lateral buds, while remaining attached to the parent, themselves give origin to buds; and so produce a branched aggregate of worms. How shall we explain the reparative and reproductive powers thus exemplified? It seems undeniable that each portion has an ability to produce, according to circumstances, the whole creature or a missing part of the creature. When we read of Sir J. Dalyell that he "cut a Dasychone into three pieces; the hindermost produced a head, the anterior piece developed an anus, and the middle portion formed both a head and a tail" we are not furnished with an explanation by the hypothesis of gemmules or by the hypothesis of determinants; for we cannot arbitrarily assume that wherever a missing organ has to be produced there exists the needful supply of gemmules or of determinants representing that organ. The hypothesis that physiological units have everywhere a proclivity towards the organic form of the species, appears more congruous with the facts; but even this does not cover the cases in which a new worm grows from a lateral bud. The tendency to complete the individual structure might be expected rather to restrain this breaking of the lines of complete structure.

Still less explicable in any way thus far proposed are certain remedial actions seen in animals. An example of them was furnished in § 67, where "false joints" were described--joints formed at places where the ends of a broken bone, failing to unite, remain moveable one upon the other. According to the character of the habitual motions there results a rudely formed hinge-joint or a ball-and-socket joint, either having the various constituent parts--periosteum, fibrous tissue, capsule, ligaments. Now Mr. Darwin's hypothesis, contemplating only normal structures, fails to account for this formation of an abnormal structure. Neither can we ascribe this local development to determinants: there were no appropriate ones in the germ-plasm, since no such structure was provided for. Nor does the hypothesis of physiological units, as presented in preceding chapters, yield an interpretation. These could have no other tendency than to restore the normal form of the limb, and might be expected to oppose the genesis of these new parts.

Thus we have to seek, if not another hypothesis, then some such qualification of an existing hypothesis as will harmonize it with various exceptional phenomena.

§ 97d. In Part II of the Principles of Sociology, published in 1876, will be found elaborated in detail that analogy between individual organization and social organization which was briefly sketched out in an essay on "The Social Organism" published in 1860. In §§ 241-3 a parallel is drawn between the developments of the sustaining systems of the two; and it is pointed out how, in the one case as in the other, the components--here organic units and there citizens--have their activities and arrangements mainly settled by local conditions. One leading example is that the parts constituting the alimentary canal, while jointly fitted to the nature of the food, are severally adapted to the successive stages at which the food arrives in its progress; and that in an analogous way the industries carried on by peoples forming different parts of a society, are primarily determined by the natures of things around--agriculture, pastoral and arable, special manufactures and minings, ship-building and fishing: the respective groups falling into fit combinations and becoming partially modified to suit their work. The implication is that while the organization of a society as a whole depends on the characters of its units, in such way that by some types of men despotisms are always evolved while by other types there are evolved forms of government partially free--forms which repeat themselves in colonies--there is, on the other hand, in every case a local power of developing appropriate structures. And it might have been pointed out that similarly in types of creatures not showing much consolidation, as the Annelida, many of the component divisions, largely independent in their vitalities, are but little affected in their structures by the entire aggregate.

My purpose at that time being the elucidation of sociological truths, it did not concern me to carry further the biological half of this comparison. Otherwise there might have been named the case in which a supernumerary finger, beginning to bud out, completes itself as a local organ with bones, muscles, skin, nail, etc., in defiance of central control: even repeating itself when cut off. There might also have been instanced the above-named formation of a false joint with its appurtenances. For the implication in both cases is that a local group of units, determined by circumstances towards a certain structure, coerces its individual units into that structure.

Now let us contemplate the essential fact in the analogy. The men in an Australian mining-camp, as M. Pierre Leroy Beaulieu points out, fall into Anglo-Saxon usages different from those which would characterize a French mining-camp. Emigrants to a far West settlement in America quickly establish post-office, bank, hotel, newspaper, and other urban institutions. We are thus shown that along with certain traits leading to a general type of social organization, there go traits which independently produce fit local organizations. Individuals are led into occupations and official posts, often quite new to them, by the wants of those around--are now influenced and now coerced into social arrangements which, as shown perhaps by gambling saloons, by shootings at sight, and by lynchings, are scarcely at all affected by the central government. Now the physiological units in each species appear to have a similar combination of capacities. Besides their general proclivity towards the specific organization, they show us abilities to organize themselves locally; and these abilities are in some cases displayed in defiance of the general control, as in the supernumerary finger or the false joint. Apparently each physiological unit, while having in a manner the whole organism as the structure which, along with the rest, it tends to form, has also an aptitude to take part in forming any local structure, and to assume its place in that structure under the influence of adjacent physiological units.

A familiar fact supports this conclusion. Everyone has at hand, not figuratively but literally, an illustration. Let him compare the veins on the backs of his two hands, either with one another or with the veins on another person's hands, and he will see that the branchings and inosculations do not correspond: there is no fixed pattern. But on progressing inwards from the extremities, the distribution of the veins becomes settled--there is a pattern-arrangement common to all persons. These facts imply a predominating control by adjacent parts where control by the aggregate is less easy. A constant combination of forces which, towards the centre, produces a typical structure, fails to do this at the periphery where, during development, the play of forces is less settled. This peripheral vascular structure, not having become fixed because one arrangement is as good as another, is in each determined by the immediately surrounding influences.

§ 97e. And now let us contemplate the verifications which recent experiments have furnished--experiments made by Prof. G. Born of Breslau, confirming results earlier reached by Vulpian and adding more striking results of kindred nature. They leave no longer doubtful the large share taken by local organizing power as distinguished from central organizing power.

The independent vitality shown by separated portions of ventral skin from frog-larvæ may be named as the first illustration. With their attached yolk-cells these lived for days, and underwent such transformations as proved some structural proclivity, though of course the product was amorphous. Detached portions of tails of larvæ went on developing their component parts in much the same ways as they would have done if remaining attached. More striking still was the evidence furnished by experiments in grafting. These proved that the undifferentiated rudiment of an organ will, when cut off and joined to a non-homologous place in another individual, develop itself as it would have done if left in its original place. In brief, then, we may say that each part is in chief measure autogenous.

These strange facts presented by small aggregates of organic matter, which are the seats of extremely complex forces, will seem less incomprehensible if we observe what has taken place in a vast aggregate of inorganic matter which is the seat of very simple forces--the Solar System. Transcendently different as this is in all other respects, it is analogous in the respect that, as factors of local structures, local influences predominate over the influences of the aggregate. For while the members of the Solar System, considered as a whole, are subordinate to the totality of its forces, the arrangements in each part of it are produced almost wholly by the play of forces in that part. Though the Sun affects the motions of the Moon, and though during the evolution of the Earth-and-Moon system the Sun exercised an influence, yet the relations of our world and its satellite in respect of masses and motions were in the main locally determined. Still more clearly was it thus with Jupiter and his satellites or Saturn with his rings and satellites. Remembering that the ultimate units of matter of which the Solar System is composed are of the same kinds, and that they act on one another in conformity with the same laws, we see that, remote as the case is from the one we are considering in all other respects, it is similar in the respect that during organization the energies in each locality work effects which are almost independent of the effects worked by the general energies. In this vast aggregate, as in the minute aggregates now in question, the parts are practically autogenous.

Having thus seen that in a way we have not hitherto recognized the same general principles pervade inorganic and organic evolution, let us revert to the case of super-organic evolution from which a parallel was drawn above. As analogous to the germinal mass of units out of which a new organism is to evolve, let us take an assemblage of colonists not yet socially organized but placed in a fertile region--men derived from a society (or rather a succession of societies) of long-established type, who have in their adapted natures the proclivity towards that type. In passing from its wholly unorganized state to an organized state, what will be the first step? Clearly this assemblage, though it may have within the constitutions of its units the potentialities of a specific structure, will not develop forthwith the details of that structure. The inherited natures of its units will first show themselves by separating into large groups devoted to strongly-distinguished occupations. The great mass, dispersing over promising lands, will make preparations for farming. Another considerable portion, prompted by the general needs, will begin to form a cluster of habitations and a trading centre. Yet a third group, recognizing the demand for wood, alike for agricultural and building purposes, will betake themselves to the adjacent forests. But in no case will the primary assemblage, before these separations, settle the arrangements and actions of each group: it will leave each group to settle them for itself. So, too, after these divisions have arisen. The agricultural division will not as a whole prescribe the doings of its members. Spontaneous segregation will occur: some going to a pastoral region and some to a tract which promises good crops. Nor within each of these bodies will the organization be dictated by the whole. The pastoral group will separate itself into clusters who tend sheep on the hills and clusters who feed cattle on the plains. Meanwhile those who have gravitated towards urban occupations will some of them make bricks or quarry stone, while others fall into classes who build walls, classes who prepare fittings, classes who supply furniture. Then along with completion of the houses will go occupation of them by men who bake bread, who make clothing, who sell liquors, and so on. Thus each great group will go on organizing itself irrespective of the rest; the sub-groups within each will do the same; and so will the sub-sub-groups. Quite independently of the people on the hills and the plains and in the town, those in the forest will divide spontaneously into parties who cut down trees, parties who trim and saw them, parties who carry away the timbers; while every party forms for itself an organization of "butty" or "boss," and those who work under him. Similarly with the ultimate divisions--the separate families: the arrangements and apportionments of duties in each are internally determined. Mark the fact which here chiefly concerns us. This formation of a heterogeneous aggregate with its variously adapted parts, which while influenced by the whole are mainly self-formed, goes on among units of essentially the same natures, inherited from units who belonged to similar societies. And now, carrying this conception with us, we may dimly perceive how, in a developing embryo, there may take place the formation, first of the great divisions--the primary layers--then of the outlines of systems, then of component organs, and so on continually with the minor structures contained in major structures; and how each of these progressively smaller divisions develops its own organization, irrespective of the changes going on throughout the rest of the embryo. So that though all parts are composed of physiological units of the same nature, yet everywhere, in virtue of local conditions and the influence of its neighbours, each unit joins in forming the particular structure appropriate to the place. Thus conceiving the matter, we may in a vague way understand the strange facts of autogenous development disclosed by the above named experiments.

§ 97f. "But how immeasurably complex must be the physiological units which can behave thus!" will be remarked by the reader. "To be able to play all parts, alike as members of the whole and as members of this or that organ, they must have an unimaginable variety of potentialities in their natures. Each must, indeed, be almost a microcosm within a microcosm."

Doubtless this is true. Still we have a consensus of proofs that the component units of organisms have constitutions of extremely involved kinds. Contemplate the facts and their implications. (1) Here is some large division of the animal kingdom--say the Vertebrata. The component units of all its members have certain fundamental traits in common: all of them have proclivities towards formation of a vertebral column. Leaving behind the great assemblage of Fishes with its multitudinous types, each having special units of composition, we pass to the Amphibia, in the units of which there exist certain traits superposed upon the traits they have in common with those of Fishes. Through unknown links we ascend to incipient Mammalian types and then to developed Mammalian types, the units of which must have further superposed traits. Additional traits distinguish the units of each Mammalian order; and, again, those of every genus included in it; while others severally characterize the units of each species. Similarly with the varieties in each species, and the stirps in each variety. Now the ability of any component unit to carry within itself the traits of the sub-kingdom, class, order, genus, species, variety, and at the same time to bear the traits of immediate ancestors, can exist only in a something having multitudinous proximate elements arranged in innumerable ways. (2) Again, these units must be at once in some respects fixed and in other respects plastic. While their fundamental traits, expressing the structure of the type, must be unchangeable, their superficial traits must admit of modification without much difficulty; and the modified traits, expressing variations in the parents and immediate ancestors, though unstable, must be considered as capable of becoming stable in course of time. (3) Once more we have to think of these physiological units (or constitutional units as I would now re-name them) as having such natures that while a minute modification, representing some small change of local structure, is inoperative on the proclivities of the units throughout the rest of the system, it becomes operative in the units which fall into the locality where that change occurs.

But unimaginable as all this is, the facts may nevertheless in some way answer to it. As before remarked, progressing science reveals complexity within complexity--tissues made up of cells, cells containing nuclei and cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules; and if now we conclude that the unit of protoplasm is itself an inconceivably elaborate structure, we do but recognize the complexity as going still deeper. Further, if we must assume that these component units are in every part of the body acting on one another by extremely complicated sets of forces (ethereal undulations emanating from each of the constituent molecules) determining their relative positions and actions, we are warranted by the discoveries which every day disclose more of the marvellous properties of matter. When to such examples as were given in § 36e we add the example yielded by recent experiments, showing that even a piece of bread, after subjection to pressure, exhibits diamagnetic properties unlike those it previously exhibited, we cannot doubt that these complex units composing living bodies are all of them seats of energies diffused around, enabling them to act and re-act so as to modify one another's states and positions. We are shown, too, that whatever be the natures of the complex forces emanating from each, it will, as a matter of course, happen that the power of each will be relatively great in its own neighbourhood and become gradually smaller in parts increasingly remote: making more comprehensible the autogenous character of each local structure.

Whatever be their supposed natures we are compelled to ascribe extreme complexity to these unknown somethings which have the power of organizing themselves into a structure of this or that species. If gemmules be alleged, then the ability of every organ and part of an organ to vary, implies that the gemmules it gives off are severally capable of receiving minute modifications of their ordinary structures: they must have many parts admitting of innumerable relations. Supposing determinants be assumed, then in addition to the complexity which each must have to express in itself the structure of the part evolved from it, it must have the further complexity implied by every superposed modification which causes a variation of that part. And, as we have just seen, the hypothesis of physiological units does not relieve us from the need for kindred suppositions.

One more assumption seems necessary if we are to imagine how changes of structure caused by changes of function can be transmitted. Reverting to § 54d, where an unceasing circulation of protoplasm throughout an organism was inferred, we must conceive that the complex forces of which each constitutional unit is the centre, and by which it acts on other units while it is acted on by them, tend continually to remould each unit into congruity with the structures around: superposing on it modifications answering to the modifications which have arisen in those structures. Whence is to be drawn the corollary that in course of time all the circulating units,--physiological, or constitutional if we prefer so to call them--visiting all parts of the organism, are severally made bearers of traits expressing local modifications; and that those units which are eventually gathered into sperm-cells and germ-cells also bear these superposed traits.

If against all this it be urged that such a combination of structures and forces and processes is inconceivably involved, then the reply is that so astonishing a transformation as that which an unfolding organism displays cannot possibly be effected by simple agencies.

§ 97g. But now let it be confessed that none of these hypotheses serves to render the phenomena really intelligible; and that probably no hypothesis which can be framed will do this. Many problems beyond those which embryology presents have to be solved; and no solution is furnished.

What are we to say of the familiar fact that certain small organs which, with the approach to maturity, become active, entail changes of structure in remote parts--that after the testes have undergone certain final developments, the hairs on the chin grow and the voice deepens? It has been contended that certain concomitant modifications in the fluids throughout the body may produce correlated sexual traits; and there is proof that in many of the lower animals the period of sexual activity is accompanied by a special bodily state--sometimes such that the flesh becomes unwholesome and even poisonous. But a change of this kind can hardly account for a structural change in the vocal organs in Man. No hypothesis of gemmules or determinants or physiological units enables us to understand how removal of the testes prevents those developments of the larynx and vocal cords which take place if they remain.

The inadequacy of our explanations we at once see in presence of a structure like a peacock's tail-feather. Mr. Darwin's hypothesis is that all parts of every organ are continually giving off gemmules, which are consequently everywhere present in their due proportions. But a completed feather is an inanimate product and, once formed, can add to the circulating fluids no gemmules representing all its parts. If we follow Prof. Weismann we are led into an astounding supposition. He admits that every variable part must have a special determinant, and that this results in the assumption of over two hundred thousand for the four wings of a butterfly. Let us ask what must happen in the case of a peacock's feather. On looking at the eye near its end, we see that the minute processes on the edge of each lateral thread must have been in some way exactly adjusted, in colour and position, so as to fall into line with the processes on adjacent threads: otherwise the symmetrical arrangement of coloured rings would be impossible. Each of these processes, then, being an independent variable, must have had its particular determinant. Now there are about 300 threads on the shaft of a large feather, and each of them bears on the average 1,600 processes, making for the whole feather 480,000 of these processes. For one feather alone there must have been 480,000 determinants, and for the whole tail many millions. And these, along with the determinants for the detailed parts of all the other feathers, and for the variable components of all organs forming the body at large, must have been contained in the microscopic head of a spermatozoon! Hardly a credible supposition. Nor is it easy to see how we are helped by the hypothesis of constitutional units. Take the feather in its budding state and ask how the group of such units, alike in structure and perpetually multiplying while the unfolding goes on, can be supposed by their mutual actions so to affect one another as eventually to produce the symmetrically-adjusted processes which constitute the terminal eye. Imagination, whatever licence may be given, utterly fails us.

At last then we are obliged to admit that the actual organizing process transcends conception. It is not enough to say that we cannot know it; we must say that we cannot even conceive it. And this is just the conclusion which might have been drawn before contemplating the facts. For if, as we saw in the chapter on "The Dynamic Element in Life," it is impossible for us to understand the nature of this element--if even the ordinary manifestations of it which a living body yields from moment to moment are at bottom incomprehensible; then, still more incomprehensible must be that astonishing manifestation of it which we have in the initiation and unfolding of a new organism.

Thus all we can do is to find some way of symbolizing the process so as to enable us most conveniently to generalize its phenomena; and the only reason for adopting the hypothesis of physiological units or constitutional units is that it best serves this purpose.

Spotted a typo or a problem in this chapter? Tell us

XI. CLASSIFICATION

§ 98. That orderly arrangement of objects called Classification has two purposes, which, though not absolutely distinct, are distinct in great part. It may be employed to facilitate identification, or it may be employed to organize our knowledge. If a librarian places his books in the alphabetical succession of the author's names, he places them in such way that any particular book may easily be found, but not in such way that books of a given nature stand together. When, otherwise, he makes a distribution of books according to their subjects, he neglects various superficial similarities and distinctions, and groups them according to certain primary and secondary and tertiary attributes, which severally imply many other attributes--groups them so that any one volume being inspected, the general characters of all the neighbouring volumes may be inferred. He puts together in one great division all works on History; in another all Biographical works; in another all works that treat of Science; in another Voyages and Travels; and so on. Each of his great groups he separates into sub-groups; as when he puts different kinds of Literature under the heads of Fiction, Poetry, and the Drama. In some cases he makes sub-sub-groups; as when, having divided his Scientific treatises into abstract and concrete, putting in the one Logic and Mathematics and in the other Physics, Astronomy, Geology, Chemistry, Physiology, &c.; he goes on to sub-divide his books on Physics, into those which treat of Mechanical Motion, those which treat of Heat, those which treat of Light, of Electricity, of Magnetism.

Between these two modes of classification note the essential distinctions. Arrangement according to any single conspicuous attribute is comparatively easy, and is the first that suggests itself: a child may place books in the order of their sizes, or according to the styles of their bindings. But arrangement according to combinations of attributes which, though fundamental, are not conspicuous, requires analysis; and does not suggest itself till analysis has made some progress. Even when aided by the information which the author gives on his title page, it requires considerable knowledge to classify rightly an essay on Polarization; and in the absence of a title page it requires much more knowledge. Again, classification by a single attribute, which the objects possess in different degrees, may be more or less serial, or linear. Books may be put in the order of their dates, in single file; or if they are grouped as works in one volume, works in two volumes, works in three volumes, &c., the groups may be placed in an ascending succession. But groups severally formed of things distinguished by some common attribute which implies many other attributes, do not admit of serial arrangement. You cannot rationally say either that Historical Works should come before Biographical Works, or Biographical Works before Historical Works; nor of the sub-divisions of creative Literature, into Fiction, Poetry, and the Drama, can you give a good reason why any one should take precedence of the others.

Hence this grouping of the like and separation of the unlike which constitutes Classification, can reach its complete form only by slow steps. I have shown (Essays, Vol. II., pp. 145-7) that, other things equal, the relations among phenomena are recognized in the order of their conspicuousness; and that, other things equal, they are recognized in the order of their simplicity. The first classifications are sure, therefore, to be groupings of objects which resemble one another in external or easily-perceived attributes, and attributes that are not of complex characters. Those likenesses among things which are due to their possession in common of simple obvious properties, may or may not coexist with further likenesses among them. When geometrical figures are classed as curvilinear and rectilinear, or when the rectilinear are divided into trilateral, quadrilateral, &c., the distinctions made connote various other distinctions with which they are necessarily bound up; but if liquids be classed according to their visible characters--if water, alcohol, sulphuret of carbon, &c., be grouped as colourless and transparent, we have things placed together which are unlike in their essential natures. Thus, where the objects classed have numerous attributes, the probabilities are that the early classifications, based on simple and manifest attributes, unite under the same head many objects that have no resemblance in the majority of their attributes. As the knowledge of objects increases, it becomes possible to make groups of which the members have more numerous properties in common; and to ascertain what property, or combination of properties, is most characteristic of each group. And the classification eventually arrived at is of such kind that the objects in each group have more attributes in common with one another than they have in common with any excluded objects; one in which the groups of such groups are integrated on the same principle; and one in which the degrees of differentiation and integration are proportioned to the degrees of intrinsic unlikeness and likeness. And this ultimate classification, while it serves to identify the things completely, serves also to express the greatest amount of knowledge concerning the things--enables us to predicate the greatest number of facts about each thing; and by so doing implies the most precise correspondence between our conceptions and the realities.

§ 99. Biological classifications illustrate well these phases through which classifications in general pass. In early attempts to arrange organisms in some systematic manner, we see at first a guidance by conspicuous and simple characters, and a tendency towards arrangement in linear order. In successively later attempts, we see more regard paid to combinations of characters which are essential but often inconspicuous, and an abandonment of a linear arrangement for an arrangement in divergent groups and re-divergent sub-groups.

In the popular mind, plants are still classed under the heads of Trees, Shrubs, and Herbs; and this serial classing according to the single attribute of magnitude, swayed the earliest observers. They would have thought it absurd to call a bamboo, thirty feet high, a kind of grass; and would have been incredulous if told that the Hart's-tongue should be placed in the same great division with the Tree-ferns. The zoological classifications current before Natural History became a science, had divisions similarly superficial and simple. Beasts, Birds, Fishes, and Creeping-things are names of groups marked off from one another by conspicuous differences of appearance and modes of life--creatures that walk and run, creatures that fly, creatures that live in the water, creatures that crawl. And these groups were thought of in the order of their importance.

The first arrangements made by naturalists were based either on single characters or on very simple combinations of characters; as that of Clusius, and afterwards the more scientific system of Cesalpino, recognizing the importance of inconspicuous structures. Describing plant-classifications, Lindley says:--"Rivinus invented, in 1690, a system depending upon the formation of the corolla; Kamel, in 1693, upon the fruit alone; Magnol, in 1720, on the calyx and corolla; and finally, Linnæus, in 1731, on variations in the stamens and pistil." In this last system, which has been for so long current as a means of identification (regarded by its author as transitional), simple external attributes are still depended on; and an arrangement, in great measure serial, is based on the degrees in which these attributes are possessed. In 1703, some thirty years before the time of Linnæus, our countryman Ray had sketched the outlines of a more advanced system. He said that--

Plants are either Flowerless, or Flowering; and these are Dicotyledones, or Monocotyledones.

Among the minor groups which he placed under these general heads, "were Fungi, Mosses, Ferns, Composites, Cichoraceæ, Umbellifers, Papilionaceous plants, Conifers, Labiates, &c., under other names, but with limits not very different from those now assigned to them." Being much in advance of his age, Ray's ideas remained dormant until the time of Jussieu; by whom they were developed into what has become known as the Natural System: a system subsequently improved by De Candolle. Passing through various modifications in the hands of successive botanists, the Natural System is now represented by the following form, which is based upon the table of contents prefixed to Vol. II. of Prof. Oliver's translation of the Natural History of Plants, by Prof. Kerner. His first division, Myxothallophyta (= Myxomycetes), I have ventured to omit. The territory it occupies is in dispute between zoologists and botanists, and as I have included the group in the zoological classification, agreeing that its traits are more animal than vegetal, I cannot also include it in the botanical classification.

Here, linear arrangement has disappeared: there is a breaking up into groups and sub-groups and sub-sub-groups, which do not admit of being placed in serial order, but only in divergent and re-divergent order. Were there space to exhibit the way in which the Alliances are subdivided into Orders, and these into Genera, and these into Species, the same principle of co-ordination would be still further manifested.

PHYLA. CLASSES. ALLIANCES. SUB-PHYLA. SUB-CLASSES.

THALLOPHYTA I. Schizophyta 2. Cyanophyceæ. Blue-green Algæ. 3. Schizomycetes. II. Dinoflagellata Peridineæ 4. III. Bacillariales 5. IV. Gamophyceæ I. Chlorophyceæ 6. Protococcoideæ. 7. Siphoneæ. 8. Confervoideæ. 9. Conjugatæ. 10. Charales. 11. Phæophyceæ. 12. Dictyotales. 13. Florideæ, Red Seaweeds. V. Fungi I. Phycomycetes 14. Oomycetes. 15. Zygomycetes. II. Mesomycetes 16. 17. III. Mycomycetes 18. 19. Additional group of Fungi, Lichenes. ARCHEGONIATÆ I. Bryophyta 20. Hepaticæ, Liverworts. 21. Musci, Mosses. II. Pteridophyta Vas. Cryptogams 22. Filices, Ferns. 23. Hydropterides, Rhizocarps. 24. Equisetales, Horse-tails. 25. Lycopodiales, Club-mosses. PHANEROGAMIA (Flowering Plants.) GYMNOSPERMÆ I. Cycadales, Cycads 26. II. Coniferæ 27. III. Gnetales 28. ANGIOSPERMÆ I. Monocotyledons 29. Liliifloræ. 30. Scitamineæ. 31. Gynandræ. 32. Fluviales. 33. Spadicifloræ. 34. Glumifloræ. II. Dicotyledons I. Monochlamydæ 35. Centrospermæ. 36. Protiales. 37. Daphnales. 38. Santalales. 39. Rafflesiales. 40. Asarales. 41. Euphorbiales. 42. Podostemales. 43. Viridifloræ. 44. Amentales. 45. Balanophorales. II. Monopetalæ 46. Caprifoliales. 47. Asterales. 48. Campanales. 49. Ericales. 50. Vaccinales. 51. Primulales. 52. Tubifloræ. III. Polypetalæ 53. Ranales. 54. Parietales. 55. Malvales. 56. Discifloræ. 57. Crateranthæ. 58. Myrtales. 59. Melastomales. 60. Lythrales. 61. Hygrobiæ. 62. Passifloræ. 63. Pepones. 64. Cactales. 65. Ficoidales. 66. Umbellales.

On studying the definitions of these primary, secondary, and tertiary classes, it will be found that the largest are marked off from one another by some attribute which connotes sundry other attributes; that each of the smaller classes comprehended in one of these largest classes, is marked off in a similar way from the other smaller classes bound up with it; and that so, each successively smaller class has an increased number of co-existing attributes.

§ 100. Zoological classification has had a parallel history. The first attempt which we need notice, to arrange animals in such a way as to display their affinities, is that of Linnæus. He grouped them thus:[42]--

CL. 1. MAMMALIA. Ord. Primates, Bruta, Feræ, Glires, Pecora, Belluæ, Cete.

CL. 2. AVES. Ord. Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres.

CL. 3. AMPHIBIA. Ord. Reptiles, Serpentes, Nantes.

CL. 4. PISCES. Ord. Apodes, Jugulares, Thoracici, Abdominales.

CL. 5. INSECTA. Ord. Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.

CL. 6. VERMES. Ord. Intestina, Mollusca, Testacea, Lithophyta, Zoophyta.

This arrangement of classes is obviously based on apparent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, beginning with the most superior forms and ending with the most inferior forms. While the general and vague idea of perfection determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. Not only Linnæus but his opponents, who proposed other systems, were "under the impression that animals were to be arranged together into classes, orders, genera, and species, according to their more or less close external resemblance." This conception survived until the time of Cuvier. "Naturalists," says Agassiz, "were bent upon establishing one continual uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was: Natura non facit saltum. They called their system la chaine des êtres."

The classification of Cuvier, based on internal organization instead of external appearance, was a great advance. He asserted that there are four principal forms, or four general plans, on which animals are constructed; and, in pursuance of this assertion, he drew out the following scheme.

First Branch. ANIMALIA VERTEBRATA. Cl. 1. Mammalia. Cl. 2. Birds. Cl. 3. Reptilia. Cl. 4. Fishes.

Second Branch. ANIMALIA MOLLUSCA. Cl. 1. Cephalapoda. Cl. 2. Pteropoda. Cl. 3. Gasteropoda. Cl. 4. Acephala. Cl. 5. Brachiopoda. Cl. 6. Cirrhopoda.

Third Branch. ANIMALIA ARTICULATA. Cl. 1. Annelides. Cl. 2. Crustacea. Cl. 3. Arachnides. Cl. 4. Insects.

Fourth Branch. ANIMALIA RADIATA. Cl. 1. Echinoderms. Cl. 2. Intestinal Worms. Cl. 3. Acalephæ. Cl. 4. Polypi. Cl. 5. Infusoria.

But though Cuvier emancipated himself from the conception of a serial progression throughout the Animal Kingdom, sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differently-combined sets of attributes distinguishing the different sub-kingdoms, and swayed by the belief in a progressive development which was erroneously supposed to imply a linear arrangement of animals, they persisted in thrusting organic forms into a quite unnatural order. The following classification of Lamarck illustrates this.

INVERTEBRATA.

I. APATHETIC ANIMALS. } } Cl. 1. Infusoria. } Do not feel, and move only by their Cl. 2. Polypi. } excited irritability. No brain, no Cl. 3. Radiaria. } elongated medullary mass; no senses; Cl. 4. Tunicata. } forms varied; rarely articulations. Cl. 5. Vermes. }

II. SENSITIVE ANIMALS. } Feel, but obtain from their sensations } only perceptions of objects, a Cl. 6. Insects. } sort of simple ideas, which they are Cl. 7. Arachnids. } unable to combine to obtain complex Cl. 8. Crustacea. } ones. No vertebral column; a brain Cl. 9. Annelids. } and mostly an elongated medullary Cl. 10. Cirripeds. } mass; some distinct senses; muscles Cl. 11. Conchifera. } attached under the skin; form symmetrical, Cl. 12. Mollusks. } the parts being in pairs.

VERTEBRATA.

{ Feel; acquire preservable ideas; III. INTELLIGENT ANIMALS. { perform with them operations by which { they obtain others; are intelligent in Cl. 13. Fishes. { different degrees. A vertebral column; Cl. 14. Reptiles. { a brain and a spinal marrow; distinct Cl. 15. Birds. { senses; the muscles attached to the Cl. 16. Mammalia. { internal skeleton; form symmetrical, { the parts being in pairs.

Passing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to classifications which recognize another order of facts--those of development. The embryological inquiries of Von Baer led him to arrange animals as follows:--

I. Peripheric Type. (RADIATA.) Evolutio radiata. The development proceeds from a centre, producing identical parts in a radiating order.

II. Massive Type. (MOLLUSCA.) Evolutio contorta. The development produces identical parts curved around a conical or other space.

III. Longitudinal Type. (ARTICULATA.) Evolutio gemina. The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.

IV. Doubly Symmetrical Type. (VERTEBRATA.) Evolutio bigemina. The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.

Recognizing these fundamental differences in the modes of development, as answering to fundamental divisions in the animal kingdom, Von Baer shows (among the Vertebrata at least) how the minor differences which arise at successively later embryonic stages, correspond with the minor divisions.

Like the modern classification of plants, the modern classification of animals shows us the assumed linear order completely broken up. In his lectures at the Royal Institution, in 1857, Prof. Huxley expressed the relations existing among the several great groups of the animal kingdom, by placing them at the ends of four or five radii, diverging from a centre. The diagram I cannot obtain; but in the published reports of his lectures at the School of Mines the groups were arranged as on the following page. What remnant there may seem to be of linear succession in some of the sub-groups contained in it, is merely an accident of typographical convenience. Each of them is to be regarded simply as a cluster. And if Prof. Huxley had further developed the arrangement, by dispersing the sub-groups and sub-sub-groups on the same principle, there would result an arrangement perhaps not much unlike that shown on the page succeeding this.

VERTEBRATA

(Abranchiata) Mammalia Aves Reptilia (Branchiata) Amphibia Pisces.

MOLLUSCA ANNULOSA

Cephalopoda Heteropoda } Articulata. Gasteropoda-dioecia } Insecta Arachnida } Myriapoda Crustacea { Pulmonata Gasteropoda-monoecia { Pteropoda Annuloida. Lamellibranchiata Annellata Scoleidæ Echinodermata Trematoda Rotifera Tæniadæ Turbellaria Nematoidea

COELENTERATA

Hydrozoa Actinozoa.

PROTOZOA

Infusoria Spongiadæ Gregarinidæ Noctilucidæ Foraminifera Thallassicollidæ

In the woodcut, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea will be formed of the relations among the successively-subordinate groups constituting the animal kingdom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them--in their analogies though not in their homologies. No such scheme, however, can give a correct conception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface (which of course it does not), it would still be inadequate. Such relations cannot be represented in space of two dimensions, but only in space of three dimensions.

Mammalia Aves Reptilia

Actinozoa

§ 100a. Two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. Replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. Concerning the present attitude of zoologists, Prof. MacBride writes:--

"They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other.

"The phyla recognized at present are:--

(1) Protozoa. (2) Porifera (Sponges). (3) Coelenterata. (4) Echinodermata. { Cestodes. (5) Platyhelminthes { Trematodes. { Turbellaria. (6) Nemertea. (7) Nematoda. (8) Acanthocephala (Echinorhyncus). (9) Chætognatha (Sagitta). (10) Rotifera. (11) Annelida (Includes Leeches and Gephyrea, Chætifera). (12) Gephyrea, Achæta. { Tracheata (Peripatus, Myriapods, Insects). (13) Arthropods { Arachnids. { Crustacea. { Pycnogonida. (14) Mollusca. (15) Polyzoa (Including Phoronis). (16) Brachiopoda. (17) Chordata (Includes Balanoglossus and Tunicates. Some continental zoologists do not admit Balanoglossus)."

[This last phylum of course includes the Vertebrata.]

Though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. On the adjacent page I have tried to make a tentative arrangement of this kind.

At the bottom of the table I have placed together, under the name "Compound Protozoa," those kinds of aggregated Protozoa which show no differentiations among the members of groups, and are thus distinguished from Metazoa; and I have further marked the distinction by their position, which implies that from them no evolution of higher types has taken place. Respecting the naming of the sub-kingdoms, phyla, classes, orders, &c., I have not maintained entire consistency. The relative values of groups cannot be typographically expressed in a small space with a limited variety of letters. The sizes of the letters mark the classificatory ranks, and by the thickness I have rudely indicated their zoological importance. In fixing the order of subordination of groups I have been aided by the table of contents prefixed to Mr. Adam Sedgwick's Student's Text Book of Zoology and have also made use of Prof. Ray Lankester's classifications of several sub-kingdoms.

Zoantharia Ctenophora Rugosa Acalephos Alcyonaria COELENTERATA Hydromedusae Actinozoa Hydrozoa | | | |-------- NEMERTEA | | | | +----------+ Turbellaria ----------------| | PROTOZOA PLATYHELMINTHES Corticata Trematoda Gymnomyxa Cestoda | | | | Myxomycetes | Triaronia | Vobrocina Calcarea Foraminifera | PORIFERA Compound Protozoa Demospongiae Radiolaria

Let me again emphasize the fact that the relationships of these diverging and re-diverging groups cannot be expressed on a flat surface. If we imagine a laurel-bush to be squashed flat by a horizontal plane descending upon it, we shall see that sundry of the upper branches and twigs which were previously close together will become remote, and that the relative positions of parts can remain partially true only with the minor branches. The reader must therefore expect to find some of the zoological divisions which in the order of nature are near one another, shown in the table as quite distant.

§ 101. While the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. When aggregating the smallest groups into larger groups and these into groups still larger, they have adopted certain general terms expressive of the successively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in Nature. It has been taken for granted that species, genera, orders, and classes, are assemblages of definite values--that every genus is the equivalent of every other genus in respect of its degree of distinctness; and that orders are separated by lines of demarcation which are as broad in one place as another. Though this conviction is not a formulated one, the disputes continually occurring among naturalists on the questions, whether such and such organisms are specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where not avowed. Yet that differences of opinion like these arise and remain unsettled, except when they end in the establishment of sub-species, sub-genera, sub-orders, and sub-classes, sufficiently shows that the conviction is ill-based. And this is equally shown by the impossibility of obtaining any definition of the degree of difference which warrants each further elevation in the hierarchy of classes.

It is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups which are also of equivalent values; and so on. There is no à priori reason for expecting this; and there is no à posteriori evidence implying it, save that which begs the question--that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions is of the same nature as the endeavour to thrust them into linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is necessary. Doubtless, too, in reducing the facts to something like order they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember that while our successively subordinate groups have a certain general correspondence with the realities, they tacitly ascribe to the realities a regularity which does not exist.

§ 102. A general truth of much significance is exhibited in these classifications. On observing the natures of the attributes which are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characters of the greatest importance, physiologically considered; and that the characters of the successively-subordinate groups, are characters of successively-subordinate importance. The structural peculiarity in which all members of one sub-kingdom differ from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases.

We saw (§ 56), that the broadest division among the functions is the division into "the accumulation of energy (latent in food); the expenditure of energy (latent in the tissues and certain matters absorbed by them); and the transfer of energy (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." Now in the lowest animals, united under the general name Protozoa, there is either no separation of the parts performing these functions or very indistinct separation: in the Rhizopoda, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, the Infusoria, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. Similarly when we pass from simple types to compound types--from Protozoa to Metazoa. The animals known as Coelenterata are characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and the Hydrozoa and Actinozoa, which are sub-divisions of the Coelenterata, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above the Coelenterata possess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. And then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. In all of them, too, the function of expenditure is divided between a directive apparatus and an executive apparatus--a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from one another by differences in the relative positions of their component sets of organs. The habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hæmal centres above. And while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is "composed of successive segments, usually provided with limbs," but in the other, the body is not segmented, "and no true articulated limbs are ever developed."

The sub-kingdoms being thus distinguished from one another, by the presence or absence of specialized parts devoted to fundamental functions, or else by differences in the distributions of such parts, we find, on descending to the classes, that these are distinguished from one another, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes and Amphibia are unlike higher vertebrates in possessing branchiæ, either throughout life or early in life. And every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. Mammals, again, are marked off from Birds and Reptiles by the presence of mammæ, as well as by the form of the occipital condyles. Among Mammals, the next division is based on the presence or absence of a placenta. And divisions of the Placentalia are mainly determined by the characters of the organs of external action.

Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. Each secondary assemblage, included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. And so on with each lower grade.

§ 103. What interpretation is to be put on these truths of classification? We find that organic forms admit of an arrangement everywhere indicating the fact, that along with certain attributes, certain other attributes, which are not directly connected with them, always exist. How are we to account for this fact? And how are we to account for the fact that the attributes possessed in common by the largest assemblages of forms, are the most vitally-important attributes?

No one can believe that combinations of this kind have arisen fortuitously. Even supposing fortuitous combinations of attributes might produce organisms that would work, we should still be without a clue to this special mode of combination. The chances would be infinity to one against organisms which possessed in common certain fundamental attributes, having also in common numerous non-essential attributes.

Nor, again, can any one allege that such combinations are necessary, in the sense that all other combinations are impracticable. There is not, in the nature of things, a reason why creatures covered with feathers should always have beaks: jaws carrying teeth would, in many cases, have served them equally well or better. The most general characteristic of an entire sub-kingdom, equal in extent to the Vertebrata, might have been the possession of nictitating membranes; while the internal organizations throughout this sub-kingdom might have been on many different plans.

If, as an alternative, this peculiar subordination of traits which organic forms display be ascribed to design, other difficulties suggest themselves. To suppose that a certain plan of organization was fixed on by a Creator for each vast and varied group, the members of which were to have many different modes of life, and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group where some other plan would have been more appropriate, is to ascribe a very strange motive. When we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long as in the giraffe, or quite rudimentary as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? And then, when it turns out that this possession of seven cervical vertebræ is not an absolutely-universal characteristic of mammals (there is one which has eight), shall we conclude that while, in a host of cases, there was a needless adherence to a plan for the sake of consistency, there was yet, in some cases, an inconsistent abandonment of the plan? I think we may properly refuse to draw any such conclusion.

What, then, is the meaning of these peculiar relations of organic forms? The answer to this question must be postponed. Having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution.

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XII. DISTRIBUTION

§ 104. There is a distribution of organisms in Space, and there is a distribution of organisms in Time. Looking first at their distribution in Space, we observe in it two different classes of facts. On the one hand, the plants and animals of each species have their habitats limited by external conditions: they are necessarily restricted to spaces in which their vital actions can be performed. On the other hand, the existence of certain conditions does not determine the presence of organisms that are fit for them. There are many spaces perfectly adapted for life of a high order in which only life of a much lower order is found.

While, in the inevitable restriction of organisms to environments with which their natures correspond we find a negative cause of distribution, there remains to be found that positive cause whence results the presence of organisms in some places appropriate to them and their absence from other places equally appropriate or more appropriate. Let us consider the phenomena as thus classed.

§ 105. Facts which illustrate the limiting influence of surrounding conditions are abundant, and familiar to all readers. It will be needful, however, here to cite a few typical ones of each order.

The confinement of different kinds of plants and different kinds of animals, to the media for which they are severally adapted, is the broadest fact of distribution. We have extensive groups of plants that are respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are exclusively marine, while others exist only in rivers and lakes. Among animals we similarly find some classes confined to the air and others to the water; and of the water-breathers some are restricted to salt water and others to fresh water. Less conspicuous is the fact that within each of these contrasted media there are further widespread limitations. In the sea, certain organisms exist only between certain depths, and others only between other depths--the limpet and the mussel within the littoral zone, and numerous kinds at the bottom of the ocean; and on the land, there are Floras and Faunas peculiar to low regions and others peculiar to high regions. Next we have the familiar geographical limitations made by climate. There are temperatures which restrict each kind of organism between certain isothermal lines, and hygrometric states which prevent the spread of each kind of organism beyond areas having a certain humidity or a certain dryness. Besides such general limitations we find much more special limitations. Some minute vegetal forms occur only in snow. Hot springs have their peculiar Infusoria. The habitats of certain Fungi are mines or other dark places. And there are creatures unknown beyond the water contained in particular caves. After these limits to distribution imposed by physical conditions, come limits imposed by the presence or absence of other organisms. Obviously, graminivorous animals are confined within tracts which produce plants fit for them to feed on. The great carnivores cannot exist out of regions where there are creatures large enough and numerous enough to serve for prey. The needs of the sloth limit it to certain forest-covered spaces; and there can be no insectivorous bats where there are no night-flying insects. To these dependences of the relatively-superior organisms on the relatively-inferior organisms which they consume, must be added certain reciprocal dependences of the inferior on the superior. Mr. Darwin's inquiries have shown how generally the fertilization of plants is due to the agency of insects, and how certain plants, being fertilizable only by insects of certain structures, are limited to regions inhabited by insects of such structures. Conversely, the spread of organisms is often bounded by the presence of particular organisms beyond the bounds--either competing organisms or organisms directly inimical. A plant fit for some territory adjacent to its own, fails to overrun it because the territory is pre-occupied by some plant which is its superior, either in fertility or power of resisting destructive agencies; or else fails because there lives in the territory some mammal which browses on its foliage or bird which devours nearly all its seeds. Similarly, an area in which animals of a particular species might thrive, is not colonized by them because they are not fleet enough to escape some beast of prey inhabiting this area, or because the area is infested by some insect which destroys them, as the tsetse destroys the cattle in parts of Africa. Yet another more special series of limitations accompanies parasitism. There are parasitic plants that flourish only on trees of some few species, and others that have particular animals for their habitats--as the fungus which is fatal to the silk-worm, or that which so strangely grows out of a New Zealand caterpillar. Of animal-parasites various kinds lead lives involving specialities of distribution. We have kinds which use other creatures for purposes of locomotion, as the Chelonobia uses the turtle, and as a certain Actinia uses the shell inhabited by a hermit-crab. We have the parasitism in which one creature habitually accompanies another to share its prey, like the annelid which takes up its abode in a hermit-crab's shell, and snatches from the hermit-crab the morsels of food it is eating. We have again the commoner parasitism of the Epizoa--animals which attach themselves to the surfaces of other animals, and feed on their juices or on their secretions. And once more, we have the equally common parasitism of the Entozoa--creatures which live within other creatures. Besides being restricted to the bodies of the organisms it infests, each species has usually still narrower limits of distribution; in some cases the infested organisms furnish fit habitats for the parasites only in certain regions, and in other cases only when in certain constitutional states. There are more indirect modes in which the distributions of organisms affect one another. Plants of some kinds are eaten by animals only in the absence of kinds that are preferred to them; and hence the prosperity of such plants partly depends on the presence of the preferred plants. Mr. Bates has shown that some South American butterflies thrive in regions where insectivorous birds would destroy them, did they not closely resemble butterflies of another genus which are disliked by those birds. And Mr. Darwin gives cases of dependence still more remote and involved.

Such are the chief negative causes of distribution--the inorganic and organic agencies that set bounds to the spaces which organisms of each species inhabit. Fully to understand their actions we must contemplate them as working not separately but in concert. We have to regard the physical influences, varying from year to year, as now producing an extension or restriction of the habitat in this direction and now in that, and as producing secondary extensions and restrictions by their effects on other kinds of organisms. We have to regard the distribution of each species as affected not only by causes which favour multiplication of prey or of enemies within its own area, but also by causes which produce such results in neighbouring areas. We have to conceive the forces by which the limit is maintained, as including all meteorologic influences, united with the influences, direct or remote, of numerous co-existing species.

One general truth, indicated by sundry of the above illustrations, calls for special notice--the truth that all kinds of organisms intrude on one another's spheres of existence. Of the ways in which they do this the commonest is invasion of territory. That tendency which we see in the human races, to overrun and occupy one another's lands, as well as the lands inhabited by inferior creatures, is a tendency exhibited by all classes of organisms in various ways. Among them, as among mankind, there are permanent conquests, temporary occupations, and occasional raids. Every spring an inroad is made into the area which our own birds occupy, by birds from the South; and every winter the fieldfares of the North come to share the hips and haws of our hedges, and thus entail on our native birds some mortality. Besides these regularly-recurring incursions there are irregular ones; as of locusts into countries not usually visited by them, or of certain rodents which from time to time swarm into areas adjacent to their own. Every now and then an incursion ends in permanent settlement--perhaps in conquest over indigenous species. Within these few years an American water-weed has taken possession of our ponds and rivers, and to some extent supplanted native water-weeds. Of animals may be named a small kind of red ant, having habits allied to those of tropical ants, which has of late overrun many houses in London. The rat, which must have taken to infesting ships within these few centuries, furnishes a good illustration of the readiness of animals to occupy new places that are available. And the way in which vessels visiting India are cleared of the European cockroach by the kindred Blatta orientalis, shows us how these successful invasions last only until there come more powerful invaders. Animals encroach on one another's spheres of existence in further ways than by trespassing on one another's areas: they adopt one another's modes of life. There are cases in which this usurpation of habits is slight and temporary; and there are cases where it is marked and permanent. Grey crows often join gulls in picking up food between tide-marks; and gulls may occasionally be seen many miles inland, feeding in ploughed fields and on moors. Mr. Darwin has watched a fly-catcher catching fish. He says that the greater titmouse sometimes adopts the practices of the shrike, and sometimes of the nuthatch, and that some South American woodpeckers are frugivorous while others chase insects on the wing. Of habitual intrusions on the occupations of other creatures, one case is furnished by the sea-eagle, which, besides hunting the surface of the land for prey, like the rest of the hawk-tribe, often swoops down upon fish. And Mr. Darwin names a species of petrel that has taken to diving, and has a considerably modified organization. The last cases introduce a still more remarkable class of facts of kindred meaning. This intrusion of organisms on one another's modes of life goes to the extent of intruding on one another's media. The great mass of flowering plants are terrestrial, and (aside from other needs) are required to be so by their process of fructification. But there are some which live in the water, and protrude their flowers above the surface. Nay, there is a still more striking instance. At the sea-side may be found an alga a hundred yards inland, and a phænogam rooted in salt water. Among animals these interchanges of media are numerous. Nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. Water appears to be an extremely unfit medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more than one species of fly living beneath the surface of the water and coming up occasionally for air. Birds, as a class, are specially fitted for an aerial existence; but certain tribes of them have taken to an aquatic existence--swimming on the surface of the water and making continual incursions beneath it, and some kinds have wholly lost the power of flight. Among mammals, too, which have limbs and lungs implying an organization for terrestrial life, may be named kinds living more or less in the water and are more or less adapted to it. We have water-rats and otters which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in the water, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. Conversely, sundry inhabitants of the water make excursions on the land. Eels migrate at night from one pool to another. There are fish with specially-modified gills and fin-rays serving as stilts, which, when the rivers they inhabit are partially dried-up, travel in search of better quarters. And while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial.

Guided by these two classes of facts, we must regard the bounds to each species' sphere of existence as determined by the balancing of two antagonist sets of forces. The tendency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inorganic. And these expansive and repressive energies, varying continually in their respective intensities, rhythmically equilibrate each other--maintain a limit that perpetually oscillates from side to side of a certain mean.

§ 106. As implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not inconsistent with the hypothesis that each species was originally placed in the regions most favourable to it. But the absence of a species from regions that are favourable to it cannot be thus accounted for. Were plants and animals localized wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect Floras to be similar, and Faunas to be similar, where the conditions are similar; and we might expect dissimilarities among Floras and among Faunas, proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified.

Mr. Darwin says that "in the Southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35°, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the productions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate." Still more striking are the contrasts which Mr. Darwin points out between adjacent areas that are totally cut off from each other. "No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama." On opposite sides of high mountain-chains, also, there are marked differences in the organic forms--differences not so marked as where the barriers are absolutely impassable, but much more marked than are necessitated by unlikenesses of physical conditions.

Not less suggestive is the converse fact that wide geographical areas which offer decided geologic and meteorologic contrasts, are peopled by nearly-allied groups of organisms, if there are no barriers to migration. "The naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American Ostrich), and northward the plains of La Plata by another species of the same genus; and not by a true ostrich or emu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Rodents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If we look to the islands off the American shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially American."

What is the generalization implied by these two groups of facts? On the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different Faunas. On the one hand, we have areas remote from one another in latitude, and contrasted in soil as well as climate, occupied by closely-allied Faunas. Clearly then, as like organisms are not universally, or even generally, found in like habitats, nor very unlike organisms in very unlike habitats, there is no manifest pre-determined adaptation of the organisms to the habitats. The organisms do no occur in such and such places solely because they are either specially fit for those places, or more fit for them than all other organisms.

The induction under which these facts come, and which unites them with various other facts, is a totally-different one. When we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers; we are at once reminded of the general truth exemplified in the last section--the truth that each species of organism tends ever to expand its sphere of existence--to intrude on other areas, other modes of life, other media. And we are shown that through these perpetually-recurring attempts to thrust itself into every accessible habitat, each species spreads until it reaches limits which are for the time insurmountable.

§ 107. We pass now to the distribution of organic forms in Time. Geological inquiry has established the truth that during a Past of immeasurable duration, plants and animals have existed on the Earth. In all countries their buried remains are found in greater or less abundance. From comparatively small areas multitudinous different types have been exhumed. Every exploration of new areas, and every closer inspection of areas already explored, brings more types to light. And beyond question, an exhaustive examination of all exposed strata, and of all strata now covered by the sea, would disclose types immensely out-numbering those at present known. Further, geologists agree that even had we before us every kind of fossil which exists, we should still have nothing like a complete index to the past inhabitants of our globe. Many sedimentary deposits have been so altered by the heat of adjacent molten matter, as greatly to obscure the organic remains contained in them. The extensive formations once called "transition," and now re-named "metamorphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossils as they probably included have been obliterated by igneous action. And the accepted conclusion is that igneous rock has everywhere resulted from the melting-up of beds of detritus originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooling crust, have been thus destroying the records of Life, it is impossible to say; but there are strong reasons for believing that the records which remain bear but a small ratio to the records which have been destroyed. Thus we have but extremely imperfect data for conclusions respecting the distribution of organic forms in Time. Some few generalizations, however, may be regarded as established.

One is that the plants and animals now existing mostly differ from the plants and animals which have existed. Though there are species common to our present Fauna and to past Faunas, yet the facies of our present Fauna differs, more or less, from the facies of each past Fauna. On carrying out the comparison, we find that past Faunas differ from one another, and that the differences between them are proportionate to their degrees of remoteness from one another in Time, as measured by their relative positions in the sedimentary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assemblages of organic forms which have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become. The number of species and genera common to the compared assemblages, becomes smaller and smaller; and the assemblages differ more and more in their general characters. Though a species of brachiopod now extant is almost identical with a species found in Silurian strata, though between the Silurian Fauna and our own there are sundry common genera of molluscs, yet it is undeniable that there is a proportion between lapse of time and divergence of organic forms.

This divergence is comparatively slow and continuous where there is continuity in the geological formations, but is sudden, and comparatively wide, wherever there occurs a great break in the succession of strata. The contrasts which thus arise, gradually or all at once, in formations that are continuous or discontinuous, are of two kinds. Faunas of different eras are distinguished partly by the absence from the one of type's present in the other, and partly by the unlikenesses between the types common to both. Such contrasts between Faunas as are due to the appearance or disappearance of types, are of secondary significance: they possibly, or probably, do not imply anything more than migrations or extinctions. The most significant contrasts are those between successive groups of organisms of the same type. And among such, as above said, the differences are, speaking generally, small and continuous where a series of conformable strata gives proof of continued existence of the type in the locality; while they are comparatively large and abrupt where the adjacent formations are shown to have been separated by long intervals.

Another general fact, referred to by Mr. Darwin as one which palæontology has made tolerably certain, is that forms and groups of forms which have once disappeared from the Earth, do not reappear. Passing over the few species which have continued throughout the whole period geologically recorded, it may be said that each species after arising, spreading for an era, and continuing abundant for an era, eventually declines and becomes extinct; and that similarly, each genus during a longer period increases in the number of its species, and during a longer period dwindles and at last dies out. After making its exit neither species nor genus ever re-enters. The like is true even of those larger groups called orders. Four types of reptiles which were once abundant have not been found in modern formations, and do not at present exist. Though nothing less than an exhaustive examination of all strata, can prove conclusively that a type of organization when once lost is never reproduced, yet so many facts point to this inference that its truth can scarcely be doubted.

To frame a conception of the total amount and general direction of the change in organic forms during the time measured by our sedimentary series, is at present impossible--the data are insufficient. The immense contrast between the few and low forms of the earliest-known Fauna, and the many and high forms of our existing Fauna, has been commonly supposed to prove, not only great change but great progress. Nevertheless, this appearance of progress may be, and probably is, mainly illusive. Wider knowledge has shown that remains of comparatively well-organized creatures really existed in strata long supposed to be devoid of them, and that where they are absent, the nature of the strata often explains their absence, without assuming that they did not exist when these strata were formed. It is a tenable hypothesis that the successively-higher types fossilized in our successively-later deposits, indicate nothing more than successive migrations from pre-existing continents to continents that were step by step emerging from the ocean--migrations which necessarily began with the inferior orders of organisms, and included the successively-superior orders as the new lands became more accessible to them and better fitted for them.[43]

While the evidence usually supposed to prove progression is thus untrustworthy, there is trustworthy evidence that there has been, in many cases, little or no progression. Though the orders which have existed from palæozoic and mesozoic times down to the present day, are almost universally changed, yet a comparison of ancient and modern members of these orders shows that the total amount of change is not relatively great, and that it is not manifestly towards a higher organization. Though nearly all the living forms which have prototypes in early formations differ from these prototypes specially, and in most cases generically, yet ordinal peculiarities are, in numerous cases, maintained from the earliest times geologically recorded, down to our own time; and we have no visible evidence of superiority in the existing genera of these orders. In his lecture "On the Persistent Types of Animal Life," Prof. Huxley enumerated many cases. On the authority of Dr. Hooker he stated "that there are Carboniferous plants which appear to be generically identical with some now living: that the cone of the Oolitic Araucaria is hardly distinguishable from that of an existing species; that a true Pinus appears in the Purbecks and a Juglans in the chalk." Among animals he named palæozoic and mesozoic corals which are very like certain extant corals; genera of Silurian molluscs that answer to existing genera; insects and arachnids in the coal-formations that are not more than generically distinct from some of our own insects and arachnids. He instanced "the Devonian and Carboniferous Pleuracanthus, which differs no more from existing sharks than these do from one another;" early mesozoic reptiles "identical in the essential characters of their organization with those now living;" and Triassic mammals which did not differ "nearly so much from some of those which now live, as these differ from one another." Continuing the argument in his "Anniversary Address to the Geological Society" in 1862, Prof. Huxley gave many cases in which the changes that have taken place, are not changes towards a more specialized or higher organization--asking "in what sense are the Liassic Chelonia inferior to those which now exist? How are the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic or more differentiated species than those of the Lias?" While, however, contending that in most instances "positive evidence fails to demonstrate any sort of progressive modification towards a less embryonic or less generalized type in a great many groups of animals of long-continued geological existence," Prof. Huxley added that there are other groups, "co-existing with them under the same conditions, in which more or less distinct indications of such a process seem to be traceable." And in illustration of this, he named that better development of the vertebræ which characterizes some of the more modern fishes and reptiles, when compared with ancient fishes and reptiles of the same orders; and the "regularity and evenness of the dentition of the Anoplotherium as contrasting with that of existing Artiodactyles."[44]

The facts thus summed up do not show that higher forms have not arisen in the course of geologic time, any more than the facts commonly cited prove that higher forms have arisen; nor are they regarded by Professor Huxley as showing this. Were those which have survived from palæozoic and mesozoic days down to our own day, the only types; and did the modifications, rarely of more than generic value, which these types have undergone, give no better evidences of increased complexity than are actually given by them; then it would be inferable that there has been no appreciable advance. But there now exist, and have existed during the more recent geologic epochs, various types which are not known to have existed in earlier epochs--some of them widely unlike these persistent types and some of them nearly allied to these persistent types. As yet, we know nothing about the origins of these new types. But it is possible that causes like those which have produced generic differences in the persistent types, have, in some or many cases, produced modifications great enough to constitute ordinal differences. If structural contrasts not exceeding certain moderate limits are held to mark only generic distinctions; and if organisms displaying larger contrasts are regarded as ordinally or typically distinct; it is obvious that the persistence of a given type through a long geologic period without apparently undergoing deviations of more than generic value, by no means disproves the occurrence of far greater deviations in other cases; since the forms resulting from such far greater deviations, being regarded as typically distinct forms, will not be taken as evidence of great change in an original type. That which Prof. Huxley's argument proves, and that only which he considers it to prove, is that organisms have no innate tendencies to assume higher forms; and that "any admissible hypothesis of progressive modification, must be compatible with persistence without progression through indefinite periods."

One very significant fact must be added concerning the relation between distribution in Time and distribution in Space. I quote it from Mr. Darwin:--"Mr. Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the living marsupials of that continent. In South America a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of La Plata; and Professor Owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to the South American types. This relationship is even more clearly seen in the wonderland collection of fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so much impressed with these facts that I strongly insisted, in 1839 and 1845, on this 'law of the succession of types,'--on 'this wonderful relationship in the same continent between the dead and the living.' Professor Owen has subsequently extended the same generalization to the Mammals of the Old World. We see the same law in this author's restorations of the extinct and gigantic birds of New Zealand. We see it also in the birds of the caves of Brazil. Mr. Woodward has shown that the same law holds good with sea-shells, but from the wide distribution of most genera of molluscs, it is not well displayed by them. Other cases could be added, as the relation between the extinct and living landshells of Madeira, and between the extinct and living brackish-water shells of the Aralo-Caspian Sea."

The general results, then, are these. Our knowledge of distribution in Time, being derived wholly from the evidence afforded by fossils, is limited to that geologic time of which some records remain--cannot extend to those remoter times the records of which have been obliterated. From these remaining records, which probably form but a small fraction of the whole, the general facts deducible are these:--That such organic types as have lived through successive epochs, have almost universally undergone modifications of specific and generic values--modifications which have commonly been great in proportion as the period has been long. That besides the types which have persisted from ancient eras down to our own era, other types have from time to time made their appearance in the ascending series of strata--types of which some are lower and some higher than the types previously recorded; but whence these new types came, and whether any of them arose by divergence from the previously-recorded types, the evidence does not yet enable us to say. That in the course of long geologic epochs nearly all species, most genera, and a few orders, have become extinct; and that a species, genus, or order, which has once disappeared from the Earth never reappears. And, lastly, that the Fauna now occupying each separate area of the Earth's surface is very nearly allied to the Fauna which existed on that area during recent geologic times.

§ 108. Omitting sundry minor generalizations, the exposition of which would involve too much detail, what is to be said of these major generalizations?

The distribution in Space cannot be said to imply that organisms have been designed for their particular habitats and placed in them; since, besides the habitat in which each kind of organism is found there are commonly other habitats, as good or better for it, from which it is absent--habitats to which it is so much better fitted than organisms now occupying them, that it extrudes these organisms when allowed the opportunity. Neither can we suppose that the purpose has been to establish varieties of Floras and Faunas; since, if so, why are the Floras and Faunas but little divergent in widely-sundered areas between which migration is possible, while they are markedly divergent in adjacent areas between which migration is impossible?

Passing to distributions in Time, there arise the questions--why during nearly the whole of that vast period geologically recorded have there existed none of those highest organic forms which have now overrun the Earth?--how is it that we find no traces of a creature endowed with large capacities for knowledge and happiness? The answer that the Earth was not, in remote times, a fit habitation for such a creature, besides being unwarranted by the evidence, suggests the equally awkward question--why during untold millions of years did the Earth remain fit only for inferior creatures? What, again, is the meaning of extinction of types? To conclude that the saurian type was replaced by other types at the beginning of the tertiary period, because it was not adapted to the conditions which then arose, is to conclude that it could not be modified into fitness for the conditions; and this conclusion is at variance with the hypothesis that creative skill is shown in the multiform adaptations of one type to many ends.

What interpretations may rationally be put on these and other general facts of distribution in Space and Time, will be seen in the next division of this work.

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I. PRELIMINARY

§ 109. In the foregoing Part, we have contemplated the most important of the generalizations to which biologists have been led by observation of organisms; as well as some others which contemplation of the facts has suggested to me. These Inductions of Biology have also been severally glanced at on their deductive sides; for the purpose of noting the harmony existing between them and those primordial truths set forth in First Principles. Having thus studied the leading phenomena of life separately, we are prepared for studying them as an aggregate, with the view of arriving at the most general interpretation of them.

There is an ensemble of vital phenomena presented by each organism in the course of its growth, development, and decay; and there is an ensemble of vital phenomena presented by the organic world as a whole. Neither of these can be properly dealt with apart from the other. But the last of them may be separately treated more conveniently than the first. What interpretation we put on the facts of structure and function in each living body, depends entirely on our conception of the mode in which living bodies in general have originated. To form some conclusion respecting this mode--a provisional if not a permanent conclusion--must therefore be our first step.

We have to choose between two hypotheses--the hypothesis of Special Creation and the hypothesis of Evolution. Either the multitudinous kinds of organisms which now exist, and the far more multitudinous kinds which have existed during past geologic eras, have been from time to time separately made; or they have arisen by insensible steps, through actions such as we see habitually going on. Both hypotheses imply a Cause. The last, certainly as much as the first, recognizes this Cause as inscrutable. The point at issue is, how this inscrutable Cause has worked in the production of living forms. This point, if it is to be decided at all, is to be decided only by examination of evidence. Let us inquire which of these antagonist hypotheses is most congruous with established facts.

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II. GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS.[45]

§ 110. Early ideas are not usually true ideas. Undeveloped intellect, be it that of an individual or that of the race, forms conclusions which require to be revised and re-revised, before they reach a tolerable correspondence with realities. Were it otherwise there would be no discovery, no increase of intelligence. What we call the progress of knowledge, is the bringing of Thoughts into harmony with Things; and it implies that the first Thoughts are either wholly out of harmony with Things, or in very incomplete harmony with them.

If illustrations be needed the history of every science furnishes them. The primitive notions of mankind as to the structure of the heavens were wrong; and the notions which replaced them were successively less wrong. The original belief respecting the form of the Earth was wrong; and this wrong belief survived through the first civilizations. The earliest ideas that have come down to us concerning the natures of the elements were wrong; and only in quite recent times has the composition of matter in its various forms been better understood. The interpretations of mechanical facts, of meteorological facts, of physiological facts, were at first wrong. In all these cases men set out with beliefs which, if not absolutely false, contained but small amounts of truth disguised by immense amounts of error.

Hence the hypothesis that living beings resulted from special creations, being a primitive hypothesis, is probably an untrue hypothesis. It would be strange if, while early men failed to reach the truth in so many cases where it is comparatively conspicuous, they reached it in a case where it is comparatively hidden.

§ 111. Besides the improbability given to the belief in special creations, by its association with mistaken beliefs in general, a further improbability is given to it by its association with a special class of mistaken beliefs. It belongs to a family of beliefs which have one after another been destroyed by advancing knowledge; and is, indeed, almost the only member of the family surviving among educated people.

We all know that the savage thinks of each striking phenomenon, or group of phenomena, as caused by some separate personal agent; that out of this conception there grows up a polytheistic conception, in which these minor personalities are variously generalized into deities presiding over different divisions of nature; and that these are eventually further generalized. This progressive consolidation of causal agencies may be traced in the creeds of all races, and is far from complete in the creed of the most advanced races. The unlettered rustics who till our fields, do not let the consciousness of a supreme power wholly absorb the aboriginal conceptions of good and evil spirits, and of charms or secret potencies dwelling in particular objects. The earliest mode of thinking changes only as fast as the constant relations among phenomena are established. Scarcely less familiar is the truth, that while accumulating knowledge makes these conceptions of personal causal agents gradually more vague, as it merges them into general causes, it also destroys the habit of thinking of them as working after the methods of personal agents. We do not now, like Kepler, assume guiding spirits to keep the planets in their orbits. It is no longer the universal belief that the sea was once for all mechanically parted from the dry land; or that the mountains were placed where we see them by a sudden creative act. All but a narrow class have ceased to suppose sunshine and storm to be sent in some arbitrary succession. The majority of educated people have given up thinking of epidemics of punishments inflicted by an angry deity. Nor do even the common people regard a madman as one possessed by a demon. That is to say, we everywhere see fading away the anthropomorphic conception of Cause. In one case after another, is abandoned the ascription of phenomena to a will analogous to the human will, working by methods analogous to human methods.

If, then, of this once-numerous family of beliefs the immense majority have become extinct, we may not unreasonably expect that the few remaining members of the family will become extinct. One of these is the belief we are here considering--the belief that each species of organism was specially created. Many who in all else have abandoned the aboriginal theory of things, still hold this remnant of the aboriginal theory. Ask any well-informed man whether he accepts the cosmogony of the Indians, or the Greeks, or the Hebrews, and he will regard the question as next to an insult. Yet one element common to these cosmogonies he very likely retains: not bearing in mind its origin. For whence did he get the doctrine of special creations? Catechise him, and he is forced to confess that it was put into his mind in childhood, as one portion of a story which, as a whole, he has long since rejected. Why this fragment is likely to be right while all the rest is wrong, he is unable to say. May we not then expect that the relinquishment of all other parts of this story, will by and by be followed by the relinquishment of this remaining part of it?

§ 112. The belief which we find thus questionable, both as being a primitive belief and as being a belief belonging to an almost-extinct family, is a belief not countenanced by a single fact. No one ever saw a special creation; no one ever found proof of an indirect kind that a special creation had taken place. It is significant, as Dr. Hooker remarks, that naturalists who suppose new species to be miraculously originated, habitually suppose the origination to occur in some region remote from human observation. Wherever the order of organic nature is exposed to the view of zoologists and botanists, it expels this conception; and the conception survives only in connexion with imagined places, where the order of organic nature is unknown.

Besides being absolutely without evidence to give it external support, this hypothesis of special creations cannot support itself internally--cannot be framed into a coherent thought. It is one of those illegitimate symbolic conceptions which are mistaken for legitimate symbolic conceptions (First Principles, § 9), because they remain untested. Immediately an attempt is made to elaborate the idea into anything like a definite shape, it proves to be a pseud-idea, admitting of no definite shape. Is it supposed that a new organism, when specially created, is created out of nothing? If so, there is a supposed creation of matter; and the creation of matter is inconceivable--implies the establishment of a relation in thought between nothing and something--a relation of which one term is absent--an impossible relation. Is it supposed that the matter of which the new organism consists is not created for the occasion, but is taken out of its pre-existing forms and arranged into a new form? If so, we are met by the question--how is the re-arrangement effected? Of the myriad atoms going to the composition of the new organism, all of them previously dispersed through the neighbouring air and earth, does each, suddenly disengaging itself from its combinations, rush to meet the rest, unite with them into the appropriate chemical compounds, and then fall with certain others into its appointed place in the aggregate of complex tissues and organs? Surely thus to assume a myriad supernatural impulses, differing in their directions and amounts, given to as many different atoms, is a multiplication of mysteries rather than the solution of a mystery. For every one of these impulses, not being the result of a force locally existing in some other form, implies the creation of force; and the creation of force is just as inconceivable as the creation of matter. It is thus with all attempted ways of representing the process. The old Hebrew idea that God takes clay and moulds a new creature, as a potter moulds a vessel, is probably too grossly anthropomorphic to be accepted by any modern defender of the special-creation doctrine. But having abandoned this crude belief, what belief is he prepared to substitute? If a new organism is not thus produced, then in what way is one produced? or rather--in what way does he conceive a new organism to be produced? We will not ask for the ascertained mode, but will be content with a mode which can be consistently imagined. No such mode, however, is assignable. Those who entertain the proposition that each kind of organism results from a divine interposition, do so because they refrain from translating words into thoughts. They do not really believe, but rather believe they believe. For belief, properly so called, implies a mental representation of the thing believed, and no such mental representation is here possible.

§ 113. If we imagine mankind to be contemplated by some being as short-lived as an ephemeron, but possessing intelligence like our own--if we imagine such a being studying men and women, during his few hours of life, and speculating as to the mode in which they came into existence; it is manifest that, reasoning in the usual way, he would suppose each man and woman to have been separately created. No appreciable changes of structure occurring in any of them during the time over which his observations extended, this being would probably infer that no changes of structure were taking place, or had taken place; and that from the outset each man and woman had possessed all the characters then visible--had been originally formed with them. The application is obvious. A human life is ephemeral compared with the life of a species; and even the period over which the records of all human lives extend, is ephemeral compared with the life of a species. There is thus a parallel contrast between the immensely-long series of changes which have occurred during the life of a species, and that small portion of the series open to our view. And there is no reason to suppose that the first conclusion drawn by mankind from this small part of the series visible to them, is any nearer the truth than would be the conclusion of the supposed ephemeral being respecting men and women.

This analogy, suggesting as it does how the hypothesis of special creations is merely a formula for our ignorance, raises the question--What reason have we to assume special creations of species but not of individuals; unless it be that in the case of individuals we directly know the process to be otherwise, but in the case of species do not directly know it to be otherwise? Have we any ground for concluding that species were specially created, except the ground that we have no immediate knowledge of their origin? And does our ignorance of the manner in which they arose warrant us in asserting that they arose by special creation?

Another question is suggested by this analogy. Those who, in the absence of immediate evidence of the way in which species arose, assert that they arose not in a natural way allied to that in which individuals arise, but in a supernatural way, think that by this supposition they honour the Unknown Cause of things; and they oppose any antagonist doctrine as amounting to an exclusion of divine power from the world. But if divine power is demonstrated by the separate creation of each species, would it not have been still better demonstrated by the separate creation of each individual? Why should there exist this process of natural genesis? Why should not omnipotence have been proved by the supernatural production of plants and animals everywhere throughout the world from hour to hour? Is it replied that the Creator was able to make individuals arise from one another in a natural succession, but not to make species thus arise? This is to assign a limit to power instead of magnifying it. Either it was possible or not possible to create species and individuals after the same general method. To say that it was not possible is suicidal in those who use this argument; and if it was possible, it is required to say what end is served by the special creation of species which would not have been better served by the special creation of individuals. Again, what is to be thought of the fact that the immense majority of these supposed special creations took place before mankind existed? Those who think that divine power is demonstrated by special creations, have to answer the question--to whom demonstrated? Tacitly or avowedly, they regard the demonstrations as being for the benefit of mankind. But if so, to what purpose were the millions of these demonstrations which took place on the Earth when there were no intelligent beings to contemplate them? Did the Unknowable thus demonstrate his power to himself? Few will have the hardihood to say that any such demonstration was needful. There is no choice but to regard them, either as superfluous exercises of power, which is a derogatory supposition, or as exercises of power that were necessary because species could not be otherwise produced, which is also a derogatory supposition.

§ 113a. Other implications concerning the divine character must be recognized by those who contend that each species arose by divine fiat. It is hardly supposable that Infinite Power is exercised in trivial actions effecting trivial changes. Yet the organic world in its hundreds of thousands of species shows in each sub-division multitudinous forms which, though unlike enough to be classed as specifically distinct, diverge from one another only in small details which have no significance in relation to the life led. Sometimes the number of specific distinctions is so great that did they result from human agency we should call them whimsical.

For example, in Lake Baikal are found 115 species of an amphipod, Gammarus; and the multiplicity becomes startling on learning that this number exceeds the number of all other species of the genus: various as are the conditions to which, throughout the rest of the world, the genus is subject. Still stranger seems the superfluous exercise of power on examining the carpet of living forms at the bottom of the ocean. Not dwelling on the immense variety of creatures unlike in type which live miles below the surface in absolute darkness, it will suffice to instance the Polyzoa alone: low types of animals so small that a thousand of them would not cover a square inch, and on which, nevertheless, there has been, according to the view we are considering, an exercise of creative skill such that by small variations of structure more than 350 species have been produced!

Kindred illustrations are furnished by the fauna of caverns. Are we to suppose that numerous blind creatures--crustaceans, myriapods, spiders, insects, fishes--were specially made sightless to fit them for the Mammoth Cave? Or what shall we say of the Proteus, a low amphibian with rudimentary eyes, which inhabits certain caves in Carniola, Carinthia and Dalmatia and is not found elsewhere. Must we conclude that God went out of his way to devise an animal for these places?

More puzzling still is a problem presented to the special-creationist by a batrachian inhabiting Central Australia. In a region once peopled by numerous animals but now made unfit by continuous droughts, there exists a frog which, when the pools are drying up, fills itself with water and burrowing in the mud hibernates until the next rains; which may come in a year or may be delayed for two years. What is to be thought of this creature? Were its structure and the accompanying instinct divinely planned to fit it to this particular habitat?

Many such questions might be asked which, if answered as the current theory necessitates, imply a divine nature hardly like that otherwise assumed.

§ 114. Those who espouse the aboriginal hypothesis entangle themselves in yet other theological difficulties. This assumption that each kind of organism was specially designed, carries with it the implication that the designer intended everything which results from the design. There is no escape from the admission that if organisms were severally constructed with a view to their respective ends, then the character of the constructor is indicated both by the ends themselves, and the perfection or imperfection with which the organisms are fitted to them. Observe the consequences.

Without dwelling on the question recently raised, why during untold millions of years there existed on the Earth no beings endowed with capacities for wide thought and high feeling, we may content ourselves with asking why, at present, the Earth is largely peopled by creatures which inflict on one another so much suffering? Omitting the human race, whose defects and miseries the current theology professes to account for, and limiting ourselves to the lower creation, what must we think of the countless different pain-inflicting appliances and instincts with which animals are endowed? Not only now, and not only ever since men have lived, has the Earth been a scene of warfare among all sentient creatures; but palæontology shows us that from the earliest eras geologically recorded, there has been going on this universal carnage. Fossil structures, in common with the structures of existing animals, show us elaborate weapons for destroying other animals. We have unmistakable proof that throughout all past time, there has been a ceaseless devouring of the weak by the strong. How is this to be explained? How happens it that animals were so designed as to render this bloodshed necessary? How happens it that in almost every species the number of individuals annually born is such that the majority die by starvation or by violence before arriving at maturity? Whoever contends that each kind of animal was specially designed, must assert either that there was a deliberate intention on the part of the Creator to produce these results, or that there was an inability to prevent them. Which alternative does he prefer?--to cast an imputation on the divine character or to assert a limitation of the divine power? It is useless for him to plead that the destruction of the less powerful by the more powerful, is a means of preventing the miseries of decrepitude and incapacity, and therefore works beneficently. For even were the chief mortality among the aged instead of among the young, there would still arise the unanswerable question--why were not animals constructed in such ways as to avoid these evils? why were not their rates of multiplication, their degrees of intelligence, and their propensities, so adjusted that these sufferings might be escaped? And if decline of vigour was a necessary accompaniment of age, why was it not provided that the organic actions should end in sudden death, whenever they fell below the level required for pleasurable existence? Will any one who contends that organisms were specially designed, assert that they could not have been so designed as to prevent suffering? And if he admits that they could have been made so as to prevent suffering, will he assert that the Creator preferred making them in such ways as to inflict suffering?

Even as thus presented the difficulty is sufficiently great; but it appears immensely greater when we examine the facts more closely. So long as we contemplate only the preying of the superior on the inferior, some good appears to be extracted from the evil--a certain amount of life of a higher order, is supported by sacrificing a great deal of life of a lower order. So long, too, as we leave out all mortality but that which, by carrying off the least perfect members of each species, leaves the most perfect members to survive and multiply; we see some compensating benefit reached through the suffering inflicted. But what shall we say on finding innumerable cases in which the suffering inflicted brings no compensating benefit? What shall we say when we see the inferior destroying the superior? What shall we say on finding elaborate appliances for furthering the multiplication of organisms incapable of feeling, at the expense of misery to organisms capable of happiness?

Of the animal kingdom as a whole, more than half the species are parasites. "The number of these parasites," says Prof. Owen, "may be conceived when it is stated that almost every known animal has its peculiar species, and generally more than one, sometimes as many as, or even more kinds than, infest the human body." This parasitism begins among the most minute creatures and pervades the entire animal kingdom from the lowest to the highest. Even Protozoa, made visible to us only by the microscope, are infested, as is Paramoecium by broods of Sphærophrya; while in large and complex animals parasites are everywhere present in great variety. More than this is true. There are parasites upon parasites--an arrangement such that those which are torturing the creatures they inhabit are themselves tortured by indwelling creatures still smaller: looking like an ingenious accumulation of pains upon pains.

But passing over the evils thus inflicted on animals of inferior dignity, let us limit ourselves to the case of Man. The Bothriocephalus latus and the Tænia solium, are two kinds of tape-worm, which flourish in the human intestines; producing great constitutional disturbances, sometimes ending in insanity; and from the germs of the Tænia, when carried into other parts of the body, arise certain partially-developed forms known as Cysticerci, Echinococci, and Coenuri, which cause disorganization more or less extensive in the brain, the lungs, the liver, the heart, the eye, &c., often ending fatally after long-continued suffering. Five other parasites, belonging to a different class, are found in the viscera of man--the Trichocephalus, the Oxyuris, the Strongylus (two species), the Ancylostomum and the Ascaris; which, beyond that defect of nutrition which they necessarily cause, sometimes induce certain irritations that lead to complete demoralization. Of another class of entozoa, belonging to the subdivision Trematoda, there are five kinds found in different organs of the human body--the liver and gall-duct, the portal vein, the intestine, the bladder, the eye. Then we have the Trichina spiralis, which passes through one phase of its existence imbedded in the muscles and through another phase of its existence in the intestine; and which, by the induced disease Trichinosis, has lately committed such ravages in Germany as to cause a panic. To these we must add the Guinea-worm, which in some part of Africa and India makes men miserable by burrowing in their legs; and the more terrible African parasite the Bilharzia, which affects 30 per cent. of the natives on the east coast with bleeding of the bladder. From entozoa, let us pass to epizoa. There are two kinds of Acari, one of them inhabiting the follicles of the skin and the other producing the itch. There are creatures that bury themselves beneath the skin and lay their eggs there; and there are three species of lice which infest the surface of the body. Nor is this all. Besides animal parasites there are sundry vegetal parasites, which grow and multiply at our cost. The Sarcina ventriculi inhabits the stomach, and produces gastric disturbance. The Leptothrix buccalis is extremely general in the mouth, and may have something to do with the decay of teeth. And besides these there are microscopic fungi which produce ringworm, porrigo, pityriasis, thrush, &c. Thus the human body is the habitat of parasites, internal and external, animal and vegetal, numbering, if all are set down, between two and three dozen species; sundry of which are peculiar to Man, and many of which produce great suffering and not unfrequently death. What interpretation is to be put on these facts by those who espouse the hypothesis of special creations? According to this hypothesis, all these parasites were designed for their respective modes of life. They were endowed with constitutions fitting them to live by absorbing nutriment from the human body; they were furnished with appliances, often of a formidable kind, enabling them to root themselves in and upon the human body; and they were made prolific in an almost incredible degree, that their germs might have a sufficient number of chances of finding their way into the human body. In short, elaborate contrivances were combined to insure the continuance of their respective races; and to make it impossible for the successive generations of men to avoid being preyed on by them. What shall we say to this arrangement? Shall we say that "the head and crown of things," was provided as a habitat for these parasites? Shall we say that these degraded creatures, incapable of thought or enjoyment, were created that they might cause human misery? One or other of these alternatives must be chosen by those who contend that every kind of organism was separately devised by the Creator. Which do they prefer? With the conception of two antagonist powers, which severally work good and evil in the world, the facts are congruous enough. But with the conception of a supreme beneficence, this gratuitous infliction of pain is absolutely incompatible.

§ 115. See then the results of our examination. The belief in special creations of organisms arose among men during the era of profoundest darkness; and it belongs to a family of beliefs which have nearly all died out as enlightenment has increased. It is without a solitary established fact on which to stand; and when the attempt is made to put it into definite shape in the mind, it turns out to be only a pseud-idea. This mere verbal hypothesis, which men idly accept as a real or thinkable hypothesis, is of the same nature as would be one, based on a day's observation of human life, that each man and woman was specially created--an hypothesis not suggested by evidence but by lack of evidence--an hypothesis which formulates ignorance into a semblance of knowledge. Further, we see that this hypothesis, failing to satisfy men's intellectual need of an interpretation, fails also to satisfy their moral sentiment. It is quite inconsistent with those conceptions of the divine nature which they profess to entertain. If infinite power was to be demonstrated, then, either by the special creation of every individual, or by the production of species by some method of natural genesis, it would be better demonstrated than by the use of two methods, as assumed by the hypothesis. And if infinite goodness was to be demonstrated, then, not only do the provisions of organic structure, if they are specially devised, fail to demonstrate it, but there is an enormous mass of them which imply malevolence rather than benevolence.

Thus the hypothesis of special creations turns out to be worthless by its derivation; worthless in its intrinsic incoherence; worthless as absolutely without evidence; worthless as not supplying an intellectual need; worthless as not satisfying a moral want. We must therefore consider it as counting for nothing, in opposition to any other hypothesis respecting the origin of organic beings.

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III. GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS

§ 116. Just as the supposition that races of organisms have been specially created, is discredited by its origin; so, conversely, the supposition that races of organisms have been evolved, is credited by its origin. Instead of being a conception suggested and accepted when mankind were profoundly ignorant, it is a conception born in times of comparative enlightenment. Moreover, the belief that plants and animals have arisen in pursuance of uniform laws, instead of through breaches of uniform laws, is a belief which has come into existence in the most-instructed class, living in these better-instructed times. Not among those who have disregarded the order of Nature, has this idea made its appearance; but among those who have familiarized themselves with the order of Nature. Thus the derivation of this modern hypothesis is as favourable as that of the ancient hypothesis is unfavourable.

§ 117. A kindred antithesis exists between the two families of beliefs, to which the beliefs we are comparing severally belong. While the one family has been dying out the other family has been multiplying. As fast as men have ceased to regard different classes of phenomena as caused by special personal agents, acting irregularly; so fast have they come to regard these different classes of phenomena as caused by a general agency acting uniformly--the two changes being correlatives. And as, on the one hand, the hypothesis that each species resulted from a supernatural act, having lost nearly all its kindred hypotheses, may be expected soon to die; so, on the other hand, the hypothesis that each species resulted from the action of natural causes, being one of an increasing family of hypotheses, may be expected to survive.

Still greater will the probability of its survival and establishment appear, when we observe that it is one of a particular genus of hypotheses which has been rapidly extending. The interpretation of phenomena as results of Evolution, has been independently showing itself in various fields of inquiry, quite remote from one another. The supposition that the Solar System has been evolved out of diffused matter, is a supposition wholly astronomical in its origin and application. Geologists, without being led thereto by astronomical considerations, have been step by step advancing towards the conviction that the Earth has reached its present varied structure by modification upon modification. The inquiries of biologists have proved the falsity of the once general belief, that the germ of each organism is a minute repetition of the mature organism, differing from it only in bulk; and they have shown, contrariwise, that every organism advances from simplicity to complexity through insensible changes. Among philosophical politicians, there has been spreading the perception that the progress of society is an evolution: the truth that "constitutions are not made but grow," is seen to be a part of the more general truth that societies are not made but grow. It is now universally admitted by philologists that languages, instead of being artificially or supernaturally formed, have been developed. And the histories of religion, of science, of the fine arts, of the industrial arts, show that these have passed through stages as unobtrusive as those through which the mind of a child passes on its way to maturity. If, then, the recognition of evolution as the law of many diverse orders of phenomena, has been spreading; may we not say that there thence arises the probability that evolution will presently be recognized as the law of the phenomena we are considering? Each further advance of knowledge confirms the belief in the unity of Nature; and the discovery that evolution has gone on, or is going on, in so many departments of Nature, becomes a reason for believing that there is no department of Nature in which it does not go on.

§ 118. The hypotheses of Special Creation and Evolution, are no less contrasted in respect of their legitimacy as hypotheses. While, as we have seen, the one belongs to that order of symbolic conceptions which are proved to be illusive by the impossibility of realizing them in thought; the other is one of those symbolic conceptions which are more or less fully realizable in thought. The production of all organic forms by the accumulation of modifications and of divergences by the continual addition of differences to differences, is mentally representable in outline, if not in detail. Various orders of our experiences enable us to conceive the process. Let us look at one of the simplest.

There is no apparent similarity between a straight line and a circle. The one is a curve; the other is defined as without curvature. The one encloses a space; the other will not enclose a space though produced for ever. The one is finite; the other may be infinite. Yet, opposite as the two are in their characters, they may be connected together by a series of lines no one of which differs from the adjacent ones in any appreciable degree. Thus, if a cone be cut by a plane at right angles to its axis we get a circle. If, instead of being perfectly at right angles, the plane subtends with the axis an angle of 89° 59', we have an ellipse which no human eye, even when aided by an accurate pair of compasses, can distinguish from a circle. Decreasing the angle minute by minute, this closed curve becomes perceptibly eccentric, then manifestly so, and by and by acquires so immensely elongated a form so as to bear no recognizable resemblance to a circle. By continuing this process the ellipse changes insensibly into a parabola. On still further diminishing the angle, the parabola becomes an hyperbola. And finally, if the cone be made gradually more obtuse, the hyperbola passes into a straight line as the angle of the cone approaches 180°. Here then we have five different species of line--circle, ellipse, parabola, hyperbola, and straight line--each having its peculiar properties and its separate equation, and the first and last of which are quite opposite in nature, connected together as members of one series, all producible by a single process of insensible modification.

But the experiences which most clearly illustrate the process of general evolution, are our experiences of special evolution, repeated in every plant and animal. Each organism exhibits, within a short time, a series of changes which, when supposed to occupy a period indefinitely great, and to go on in various ways instead of one way, give us a tolerably clear conception of organic evolution at large. In an individual development, we see brought into a comparatively infinitesimal time, a series of metamorphoses equally great with each of those which the hypothesis of evolution assumes to have taken place during immeasurable geologic epochs. A tree differs from a seed in every respect--in bulk, in structure, in colour, in form, in chemical composition. Yet is the one changed in the course of a few years into the other: changed so gradually, that at no moment can it be said--Now the seed ceases to be and the tree exists. What can be more widely contrasted than a newly-born child and the small, semi-transparent, gelatinous spherule constituting the human ovum? The infant is so complex in structure that a cyclopædia is needed to describe its constituent parts. The germinal vesicle is so simple that it may be defined in a line. Nevertheless, nine months suffice to develop the one out of the other; and that, too, by a series of modifications so small, that were the embryo examined at successive minutes, even a microscope would not disclose any sensible changes. Aided by such facts, the conception of general evolution may be rendered as definite a conception as any of our complex conceptions can be rendered. If, instead of the successive minutes of a child's foetal life, we take the lives of successive generations of creatures--if we regard the successive generations as differing from one another no more than the foetus differs in successive minutes; our imaginations must indeed be feeble if we fail to realize in thought, the evolution of the most complex organism out of the simplest. If a single cell, under appropriate conditions, becomes a man in the space of a few years; there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race.

Doubtless many minds are so unfurnished with those experiences of Nature out of which this conception is built, that they find difficulty in forming it. Looking at things rather in their statical than in their dynamical aspects, they never realize the fact that, by small increments of modification, any amount of modification may in time be generated. The surprise they feel on finding one whom they last saw as a boy, grown into a man, becomes incredulity when the degree of change is greater. To such, the hypothesis that by any series of changes a protozoon can give origin to a mammal, seems grotesque--as grotesque as Galileo's assertion of the Earth's movement seemed to his persecutors; or as grotesque as the assertion of the Earth's sphericity seems now to the New Zealanders. But those who accept a literally-unthinkable proposition as quite satisfactory, may not unnaturally be expected to make a converse mistake.

§ 119. The hypothesis of evolution is contrasted with the hypothesis of special creations, in a further respect. It is not simply legitimate instead of illegitimate, because representable in thought instead of unrepresentable; but it has the support of some evidence, instead of being absolutely unsupported by evidence. Though the facts at present assignable in direct proof that by progressive modifications, races of organisms which are apparently distinct from antecedent races have descended from them, are not sufficient; yet there are numerous facts of the order required. Beyond all question unlikenesses of structure gradually arise among the members of successive generations. We find that there is going on a modifying process of the kind alleged as the source of specific differences: a process which, though slow, does, in time, produce conspicuous changes--a process which, to all appearance, would produce in millions of years, any amount of change.

In the chapters on "Heredity" and "Variation," contained in the preceding Part, many such facts were given, and more might be added. Although little attention has been paid to the matter until recent times, the evidence already collected shows that there take place in successive generations, alterations of structure quite as marked as those which, in successive short intervals, arise in a developing embryo--nay, often much more marked; since, besides differences due to changes in the relative sizes or parts, there sometimes arise differences due to additions and suppressions of parts. The structural modification proved to have taken place since organisms have been observed, is not less than the hypothesis demands--bears as great a ratio to this brief period, as the total amount of structural change seen in the evolution of a complex organism out of a simple germ, bears to that vast period during which living forms have existed on the Earth.

We have, indeed, much the same kind and quantity of direct evidence that all organic beings have arisen through the actions of natural causes, which we have that all the structural complexities of the Earth's crust have arisen through the actions of natural causes. Between the known modifications undergone by organisms, and the totality of modifications displayed in their structures, there is no greater disproportion than between the observed geological changes, and the totality of geological changes supposed to have been similarly caused. Here and there are sedimentary deposits now slowly taking place. At this place a shore has been greatly encroached on by the sea during recorded times; and at another place an estuary has become shallower within some generations. In one region an upheaval is going on at the rate of a few feet in a century; while in another region occasional earthquakes cause slight variations of level. Appreciable amounts of denudation by water are visible in some localities; and in other localities glaciers are detected in the act of grinding down the rocky surfaces over which they glide. But these changes are infinitesimal compared with the aggregate of changes to which the Earth's crust testifies, even in its still extant systems of strata. If, then, the small changes now being wrought on the Earth's crust by natural agencies, yield warrant for concluding that by such agencies acting through vast epochs, all the structural complexities of the Earth's crust have been produced; do not the small known modifications produced in races of organisms by natural agencies, yield warrant for concluding that by natural agencies have been produced all those structural complexities which we see in them?

The hypothesis of Evolution then, has direct support from facts which, though small in amount, are of the kind required; and the ratio which these facts bear to the generalization based on them, seems as great as is the ratio between facts and generalization which, in another case, produces conviction.

§ 120. Let us put ourselves for a moment in the position of those who, from their experiences of human modes of action, draw differences respecting the mode of action of that Ultimate Power manifested to us through phenomena. We shall find the supposition that each kind of organism was separately designed and put together, to be much less consistent with their professed conception of this Ultimate Power, than is the supposition that all kinds of organisms have resulted from one unbroken process. Irregularity of method is a mark of weakness. Uniformity of method is a mark of strength. Continual interposition to alter a pre-arranged set of actions, implies defective arrangement in those actions. The maintenance of those actions, and the working out by them of the highest results, implies completeness of arrangement. If human workmen, whose machines as at first constructed require perpetual adjustment, show their increasing skill by making their machines self-adjusting; then, those who figure to themselves the production of the world and its inhabitants by a "Great Artificer," must admit that the achievement of this end by a persistent process, adapted to all contingencies, implies greater skill than its achievement by the process of meeting the contingencies as they severally arise.

So, too, it is with the contrast under its moral aspect. We saw that to the hypothesis of special creations, a difficulty is presented by the absence of high forms of life during immeasurable epochs of the Earth's existence. But to the hypothesis of evolution, absence of them is no such obstacle. Suppose evolution, and this question is necessarily excluded. Suppose special creations, and this question can have no satisfactory answer. Still more marked is the contrast between the two hypotheses, in presence of that vast amount of suffering entailed on all orders of sentient beings by their imperfect adaptations to their conditions of life, and the further vast amount of suffering entailed on them by enemies and by parasites. We saw that if organisms were severally designed for their respective places in Nature, the inevitable conclusion is that these innumerable kinds of inferior organisms which prey on superior organisms, were intended to inflict all the pain and mortality which results. But the hypothesis of evolution involves us in no such dilemma. Slowly, but surely, evolution brings about an increasing amount of happiness. In all forms of organization there is a progressive adaptation, and a survival of the most adapted. If, in the uniform working out of the process, there are evolved organisms of low types which prey on those of higher types, the evils inflicted form but a deduction from the average benefits. The universal multiplication of the most adapted must cause the spread of those superior organisms which, in one way or other, escape the invasions of the inferior; and so tends to produce a type less liable to the invasions of the inferior. Thus the evils accompanying evolution are ever being self-eliminated. Though there may arise the question--Why could they not have been avoided? there does not arise the question--Why were they deliberately inflicted? Whatever may be thought of them, it is clear that they do not imply gratuitous malevolence.

§ 121. In all respects, then, the hypothesis of evolution contrasts favourably with the hypothesis of special creation. It has arisen in comparatively-instructed times and in the most cultivated class. It is one of those beliefs in the uniform concurrence of phenomena, which are gradually supplanting beliefs in their irregular and arbitrary concurrence; and it belongs to a genus of these beliefs which has of late been rapidly spreading. It is a definitely-conceivable hypothesis; being simply an extension to the organic world at large, of a conception framed from our experiences of individual organisms; just as the hypothesis of universal gravitation was an extension of the conception which our experiences of terrestrial gravitation had produced. This definitely-conceivable hypothesis, besides the support of numerous analogies, has the support of direct evidence. We have proof that there is going on a process of the kind alleged; and though the results of this process, as actually witnessed, are minute in comparison with the totality of results ascribed to it, yet they bear to such totality a ratio as great as that by which an analogous hypothesis is justified. Lastly, that sentiment which the doctrine of special creations is thought necessary to satisfy, is much better satisfied by the doctrine of evolution; since this doctrine raises no contradictory implications respecting the Unknown Cause, such as are raised by the antagonist doctrine.

And now, having observed how, under its most general aspects, the hypothesis of organic evolution commends itself to us by its derivation, by its coherence, by its analogies, by its direct evidence, by its implications; let us go on to consider the several orders of facts which yield indirect support to it. We will begin by noting the harmonies between it and sundry of the inductions set forth in Part II.

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IV. THE ARGUMENTS FROM CLASSIFICATION

§ 122. In § 103, we saw that the relations which exist among the species, genera, orders, and classes of organisms, are not interpretable as results of any such causes as have usually been assigned. We will here consider whether they are interpretable as the results of evolution. Let us first contemplate some familiar facts.

The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form together a group of Scandinavian races, which are but slightly divergent in their characters. Welsh, Irish, and Highlanders, though they have differences, have not such differences as hide a decided community of nature: they are classed together as Celts. Between the Scandinavian race as a whole and the Celtic race as a whole, there is a distinction greater than that between the sub-divisions which make up the one or the other. Similarly, the several peoples inhabiting Southern Europe are more nearly allied to one another, than the aggregate they form is allied to the aggregates of Northern peoples. If, again, we compare these European varieties of Man, taken as a group, with that group of Eastern varieties which had a common origin with it, we see a stronger contrast than between the groups of European varieties themselves. And once more, ethnologists find differences of still higher importance between the Aryan stock as a whole and the Mongolian stock as a whole, or the Negro stock as a whole. Though these contrasts are partially obscured by intermixtures, they are not so much obscured as to hide the truths that the most-nearly-allied varieties of Man are those which diverged from one another at comparatively-recent periods; that each group of nearly-allied varieties is more strongly contrasted with other such groups that had a common origin with it at a remoter period; and so on until we come to the largest groups, which are the most strongly contrasted, and of whose divergence no trace is extant.

The relations existing among the classes and sub-classes of languages, have been briefly referred to by Mr. Darwin in illustration of his argument. We know that languages have arisen by evolution. Let us then see what grouping of them evolution has produced. On comparing the dialects of adjacent counties in England, we find that their differences are so small as scarcely to distinguish them. Between the dialects of the Northern counties taken together, and those of the Southern counties taken together, the contrast is stronger. These clusters of dialects, together with those of Scotland and Ireland, are nevertheless so similar that we regard them as one language. The several languages of Scandinavian Europe, including English, are much more unlike one another than are the several dialects which each of them includes; in correspondence with the fact that they diverged from one another at earlier periods than did their respective dialects. The Scandinavian languages have nevertheless a certain community of character, distinguishing them as a group from the languages of Southern Europe; between which there are general and special affinities that similarly unite them into a group formed of sub-groups containing sub-sub-groups. And this wider divergence between the order of languages spoken in Northern Europe and the order of languages spoken in Southern Europe, answers to the longer time that has elapsed since their differentiation commenced. Further, these two orders of modern European languages, as well as Latin and Greek and certain extinct and spoken languages of the East, are shown to have traits in common which unite them into one great class known as Aryan languages; radically distinguished from the classes of languages spoken by the other main divisions of the human race.

§ 123. Now this kind of subordination of groups which we see arises in the course of continuous descent, multiplication, and divergence, is just the kind of subordination of groups which plants and animals exhibit: it is just the kind of subordination which has thrust itself on the attention of naturalists in spite of pre-conceptions.

The original idea was that of arrangement in linear order. We saw that even after a considerable acquaintance with the structures of organisms had been acquired, naturalists continued their efforts to reconcile the facts with the notion of a uni-serial succession. The accumulation of evidence necessitated the breaking up of the imagined chain into groups and sub-groups. Gradually there arose the conviction that these groups do not admit of being placed in a line. And the conception finally arrived at, is that of certain great sub-kingdoms, very widely divergent, each made up of classes much less divergent, severally containing orders still less divergent; and so on with genera and species.

Hence this "grand fact in natural history of the subordination of group under group, which from its familiarity does not always sufficiently strike us," is perfectly in harmony with the hypothesis of evolution. The extreme significance of this kind of relation among organic forms is dwelt on by Mr. Darwin, who shows how an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general, and which is explained on the supposition of a genealogical tree by which all organisms are affiliated. If, wherever we can trace direct descent, multiplication, and divergence, this formation of groups within groups takes place; there results a strong presumption that the groups within groups which constitute the animal and vegetal kingdoms, have arisen by direct descent, multiplication, and divergence--that is, by evolution.

§ 124. Strong confirmation of this inference is yielded by the fact, that the more marked differences which divide groups are, in both cases, distinguished from the less marked differences which divide sub-groups, by this, that they are not simply greater in degree, but they are more radical in kind. Objects, as the stars, may present themselves in small clusters, which are again more or less aggravated into clusters of clusters, in such manner that the individuals of each simple cluster are much closer together than are the simple clusters gathered into a compound cluster: in which case, the trait that unites groups of groups differs from the trait that unites groups, not in nature but only in amount. But this is not so either with the groups and sub-groups which we know have resulted from evolution, or with those which we here infer have resulted from evolution. In both cases the highest or most general classes, are marked off from one another by fundamental differences that have no common measure with the differences that mark off small classes. Observe the parallelism.

We saw that each sub-kingdom of animals is distinguished from other sub-kingdoms, by some unlikeness in its main plan of organization; such as the presence or absence of a peri-visceral cavity. Contrariwise, the members of the smallest groups are united together, and separated from the members of other small groups, by modifications which do not affect the relations of essential parts. That this is just the kind of arrangement which results from evolution, the case of languages will show.

On comparing the dialects spoken in different parts of England, we find scarcely any difference but those of pronunciation: the structures of the sentences are almost uniform. Between English and the allied modern languages there are divergences of structure: there are some unlikenesses of idiom; some unlikenesses in the ways of modifying the meanings of verbs; and considerable unlikenesses in the uses of genders. But these unlikenesses are not sufficient to hide a general community of organization. A greater contrast of structure exists between these modern languages of Western Europe, and the classic languages. Differentiation into abstract and concrete elements, which is shown by the substitution of auxiliary words for inflections, has produced a higher specialization, distinguishing these languages as a group from the older languages. Nevertheless, both the ancient and modern languages of Europe, together with some Eastern languages derived from the same original, have, under all their differences of organization, a fundamental likeness; since in all of them words are formed by such a coalescence and integration of roots as destroys the independent meanings of the roots. These Aryan languages, and others which have the amalgamate character, are united by it into a class distinguished from the aptotic and agglutinate languages; in which the roots are either not united at all, or so incompletely united that one of them still retains its independent meaning. And philologists find that these radical traits which severally determine the grammatical forms, or modes of combining ideas, characterize the primary divisions among languages.

So that among languages, where we know that evolution has been going on, the greatest groups are marked off from one another by the strongest structural contrasts; and as the like holds among groups of organisms, there results a further reason for inferring that these have been evolved.

§ 125. There is yet another parallelism of like meaning. We saw (§ 101) that the successively-subordinate groups--classes, orders, genera, and species--into which zoologists and botanists segregate animals and plants, have not, in reality, those definite values conventionally given to them. There are well-marked species, and species so imperfectly marked that some systematists regard them as varieties. Between genera strong contrasts exist in many cases, and in other cases contrasts so much less decided as to leave it doubtful whether they imply generic distinctions. So, too, is it with orders and classes: in some of which there have been introduced sub-divisions, having no equivalents in others. Even of the sub-kingdoms the same truth holds. The contrast between the Coelenterata and the Mollusca, is far less than that between the Coelenterata and the Vertebrata.

Now just this same indefiniteness of value, or incompleteness of equivalence, is observable in those simple and compound and re-compound groups which we see arising by evolution. In every case the endeavour to arrange the divergent products of evolution, is met by a difficulty like that which would meet the endeavour to classify the branches of a tree, into branches of the first, second, third, fourth, &c., orders--the difficulty, namely, that branches of intermediate degrees of composition exist. The illustration furnished by languages will serve us once more. Some dialects of English are but little contrasted; others are strongly contrasted. The alliances of the several Scandinavian tongues with one another are different in degree. Dutch is much less distinct from German than Swedish is; while between Danish and Swedish there is so close a kinship that they might almost be regarded as widely-divergent dialects. Similarly on comparing the larger divisions, we see that the various languages of the Aryan stock have deviated from their original to very unlike distances. The general conclusion is manifest. While the kinds of human speech fall into groups, and sub-groups, and sub-sub-groups; yet the groups are not equal to one another in value, nor have the sub-groups equal values, nor the sub-sub-groups.

If, then, when classified, organisms fall into assemblages such that those of the same grade are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly indefinite; there is additional reason for inferring that organisms are products of evolution.

§ 126. A fact of much significance remains. If groups of organic forms have arisen by divergence and re-divergence; and if, while the groups have been developing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type; then it is inferable that there once existed greater structural likenesses between the members of allied groups than exists now. This, speaking generally, proves to be so.

Between the sub-kingdoms the gaps are extremely wide; but such distant kinships as may be discerned, bear out anticipation. Thus in the formation of the germinal layers there is a general agreement among them; and there is a further agreement among sundry of them in the formation of a gastrula. This simplest and earliest likeness, significant of primitive kinship, is in most cases soon obscured by divergent modes of development; but sundry sub-kingdoms continue to show relationships by the likenesses of their larval forms; as we see in the trochophores of the Polyzoa, Annelida, and Mollusca--sub-kingdoms the members of which by their later structural changes are rendered widely unlike.

More decided approximations exist between the lower members of classes. In tracing down the Crustacea and the Arachnida from their more complex to their simpler forms, zoologists meet with difficulties: respecting some of these simpler forms, it becomes a question which class they belong to. The Lepidosiren, about which there have been disputes whether it is a fish or an amphibian, is inferior, in the organization of its skeleton, to the great majority of both fishes and amphibia. Widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess.

Now since this kind of relationship of groups is not accounted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it; we must include it among the supports of this hypothesis which the facts of classification furnish.

§ 127. What shall we say of these leading truths when taken together? That naturalists have been gradually compelled to arrange organisms in groups within groups, and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. That while the smallest groups are the most nearly related, there exist between the great sub-kingdoms, structural contrasts of the profoundest kind, cannot but impress us as remarkable, when we see that where it is known to take place evolution actually produces these feebly-distinguished small groups, and these strongly-distinguished great groups. The impression made by these two parallelisms, which add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both--the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional types; so between the groups, sub-groups, and sub-sub-groups, which we know to have been evolved, types of intermediate values exist. And these three correspondences between the known results of evolution and the results here ascribed to evolution, have further weight given to them by the fact, that the kinship of groups through their lowest members is just the kinship which the hypothesis of evolution implies.

Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strikingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr. Darwin, "that propinquity of descent--the only known cause of the similarity of organic beings--is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classifications." When we consider that this only known cause of similarity, joined with the only known cause of divergence (the influence of conditions), gives us a key to these likenesses obscured by unlikenesses; we shall see that were there none of those remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion.

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V. THE ARGUMENTS FROM EMBRYOLOGY

§ 127a. Already I have emphasized the truth that Nature is always more complex than we suppose (§ 74a)--that there are complexities within complexities. Here we find illustrated this truth under another aspect. When seeking to formulate the arguments from Embryology, we are shown that the facts as presented in Nature are not to be expressed in the simple generalizations we at first make.

While we recognize this truth we must also recognize the truth that only by enunciation and acceptance of imperfect generalizations can we progress to perfect ones. The order of Evolution is conformed to by ideas as by other things. The advance is, and must be, from the indefinite to the definite. It is impossible to express the totality of any natural phenomenon in a single proposition. To the primary statement expressing that which is most dominant have to be added secondary statements qualifying it. We see this even in so simple a case as the flight of a projectile. The young artillery officer is first taught that a cannon-shot describes a curve treated as a parabola, though literally part of an extremely eccentric ellipse not distinguishable from a parabola. Presently he learns that atmospheric resistance, causing a continual decrease of velocity, entails a deviation from that theoretical path which is calculated on the supposition that the velocity is uniform; and this incorrectness he has to allow for. Then, further, there comes the lateral deviation due to wind, which may be appreciable if the wind is strong and the range great. To introduce him all at once to the correct conception thus finally reached would be impossible: it has to be reached through successive qualifications. And that which holds even in this simple case necessarily holds more conspicuously in complex cases.

The title of the chapter suggests a metaphor, which is, indeed, something more than a metaphor. There is an embryology of conceptions. That this statement is not wholly a figure of speech, we shall see on considering that cerebral organization is a part of organization at large; and that the evolving nervous plexus which is the correlative of an evolving conception, must conform to the general law of change conformed to in the evolution of the whole nervous structure as well as in the evolution of the whole bodily structure. As the body has at first a rude form, very remotely suggesting that which is presently developed by the superposing of modifications on modifications; so the brain as a whole and its contained ideas together make up an inner world answering with extreme indefiniteness to that outer world to which it is brought by successive approximations into tolerable correspondence; and so any nervous plexus and its associated hypothesis, which refer to some external group of phenomena under investigation, have to reach their final developments by successive corrections.

This being the course of discovery must also be the course of exposition. In pursuance of this course we may therefore fitly contemplate that early formula of embryological development which we owe to von Baer.

§ 128. Already in § 52, where the generalization of von Baer respecting the relations of embryos was set forth, there was given the warning, above repeated with greater distinctness, that it is only an adumbration.

In the words of his translator, he "found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms; that at each subsequent stage, traits are acquired which successively distinguished the developing embryo from groups of embryos that it previously resembled--thus step by step diminishing the class of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member."

Assuming for a moment that this generalization is true as it stands, or rather, assuming that the qualifications needed are not such as destroy its correspondence with the average facts, we shall see that it has profound significance. For if we follow out in thought the implications--if we conceive the germs of all kinds of organisms simultaneously developing, and imagine that after taking their first step together, at the second step one half of the vast multitude diverges from the other half; if, at the next step, we mentally watch the parts of each great assemblage beginning to take two or more routes of development; if we represent to ourselves such bifurcations going on, stage after stage, in all the branches; we shall see that there must result an aggregate analogous, in its arrangement of parts, to a tree. If this vast genealogical tree be contemplated as a whole, made up of trunk, main branches, secondary branches, and so on as far as the terminal twigs; it will be perceived that all the various kinds of organisms represented by these terminal twigs, forming the periphery of the tree, will stand related to one another in small groups, which are united into groups of groups, and so on. The embryological tree, expressing the developmental relations of organisms, will be similar to the tree which symbolizes their classificatory relations. That subordination of classes, orders, genera, and species, to which naturalists have been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. On the hypothesis of evolution this parallelism has a meaning--indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. But on any other hypothesis the parallelism is meaningless; or rather, it raises a difficulty; since it implies either an effect without a cause or a design without a purpose.

§ 129. This conception of a tree, symbolizing the relationships of types and a species derived from the same root, has a concomitant conception. The implication is that each organism, setting out from the simple nucleated cell, must in the course of its development follow the line of the trunk, some main branch, some sub-branch, some sub-sub-branch, &c., of this embryological tree; and so on till it reaches that ultimate twig representing the species of which it is a member. It must in a general way go through the particular line of forms which preceded it in all past times: there must be what has been aptly called a "recapitulation" of the successive ancestral structures. This, at least, is the conclusion necessitated by the generalization we are considering under its original crude form.

Von Baer lived in the days when the Development Hypothesis was mentioned only to be ridiculed, and he joined in the ridicule. What he conceived to be the meaning of these groupings of organisms and these relations among their embryological histories, is not obvious. The only alternative to the hypothesis of Evolution is the hypothesis of Special Creation; and as he did not accept the one it is inferable that he accepted the other. But if he did this he must in the first place have found no answer to the inquiry why organisms specially created should have the embryological kinships he described. And in the second place, after discovering that his alleged law was traversed by many and various nonconformities, he would have been without any explanation of these. Observe the positions which were open to him and the reasons which show them to be untenable.

If it be said that the conditions of the case necessitated the derivation of all organisms from simple germs, and therefore necessitated a morphological unity in their primitive states; there arises the obvious answer, that the morphological unity thus implied, is not the only morphological unity to be accounted for. Were this the only unity, the various kinds of organisms, setting out from a common primordial form, should all begin from the first to diverge individually, as so many radii from a centre; which they do not. If, otherwise, it be said that organisms were framed upon certain types, and that those of the same type continue developing together in the same direction, until it is time for them to begin putting on their specialities of structure; then the answer is, that when they do finally diverge they ought severally to develop in direct lines towards their final forms. No reason can be assigned why, having parted company, some should progress towards their final forms by irregular or circuitous routes. On the hypothesis of design such deviations are inexplicable.

The hypothesis of evolution, however, while it pre-supposes those kinships among embryos in their early forms which are found to exist, also leads us to expect nonconformities in their courses of development. If, as any rational theory of evolution implies, the progressive differentiations of types from one another during past times, have resulted from the direct and indirect effects of external conditions--if races of organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adaptations resulting from preservation of the individuals most fitted for such habits of life, or by both; and if most embryonic changes are significant of changes that were undergone by ancestral races; then these irregularities must be anticipated. For the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. In some cases they must have been more numerous than in others; in some cases they must have been greater in degree than in others; in some cases they must have been to simpler modes, in some cases to more complex modes, and in some cases to modes neither higher nor lower. Of two cognate races which diverged in the remote past, the one may have had descendants that have remained tolerably constant in their habits, while the other may have had descendants that have passed through widely-aberrant modes of life; and yet some of these last may have eventually taken to modes of life like those of the other races derived from the same stock. And if the metamorphoses of embryos indicate, in a general way, the changes of structure undergone by ancestors; then, the later embryologic changes of such two allied races will be somewhat different, though they may end in very similar forms. An illustration will make this clear. Mr. Darwin says: "Petrels are the most aërial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when unwillingly it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a petrel, but with many parts of its organization profoundly modified." Now if we suppose these grebe-like habits to be continued through a long epoch, the petrel-form to be still more obscured, and the approximation to the grebe-form still closer; it is manifest that while the chicks of the grebe and the Puffinuria will, during their early stages of development, display that likeness involved by their common derivation from some early type of bird, the chick of the Puffinuria will eventually begin to show deviations, representative of the ancestral petrel-structure, and will afterwards begin to lose these distinctions and assume the grebe-structure.

Hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different; and remembering that these intrusions have been going on from the beginning; we shall be prepared to find that the general law of embryonic parallelism is qualified by irregularities which are mostly small, in many cases considerable, and occasionally great. The hypothesis of evolution accounts for these: it does more--it implies the necessity of them.

§ 130. The substitutions of organs and the suppressions of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. Some embryos, during early stages of development, possess organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And in other embryos organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption.

We have a remarkable instance of substitution in the temporary appliances for respiration, which some embryos exhibit. During the first phase of its development, the mammalian embryo possesses a system of blood-vessels distributed over what is called the area vasculosa--a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. Now since this system of blood-vessels, not being in proximity to an oxygenated medium, cannot be serviceable to the mammalian embryo during development of the lungs, as it is serviceable in the embryo-fish during development of the gills, this needless formation of it is unaccountable as a result of design. But it is quite congruous with the supposition that the mammalian type arose out of lower vertebrate types. For in such case the mammalian embryo, passing through states representing in a general way those which its remote ancestors had in common with the lower Vertebrata, develops this system of vessels in like manner with them. An instance more significant still is furnished by certain Amphibia. One of the facts early made familiar to the natural-history student is that the tadpole breathes by external branchiæ, and that these, needful during its aquatic life, dwindle away as fast as it develops the lungs fitting it for terrestrial life. But in one of the higher Amphibia, the viviparous Salamander, these transformations ordinarily undergone during the free life of the larva, are undergone by the embryo in the egg. The branchiæ are developed though there is no use for them: lungs being substituted as breathing appliances before the creature is born.

Even more striking than the substitutions of organs are the suppressions of organs. Mr. Darwin names some cases as "extremely curious; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads;... It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." Irreconcilable with any teleological theory, these facts do not even harmonize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted; seeing that the disappearance of these incipient organs during foetal life spoils the typical resemblance. But while to other hypotheses these facts are stumbling-blocks, they yield strong support to the hypothesis of evolution.

Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, become fixed, lose, in their adult states, the limbs and senses they had when young. It may be alleged, however, that these creatures could not secure the habitats needful for them, without possessing, during their larval stages, eyes and swimming appendages which eventually become useless; that though, by losing these, their organization retrogresses in one direction, it progresses in another direction; and that, therefore, they do not exhibit the needless development of a higher type on the way to a lower type. Nevertheless there are instances of a descent in organization, following an apparently-superfluous ascent. Mr. Darwin says that in some genera of cirripedes, "the larvæ become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction."

§ 130a. But now let us contemplate more closely the energies at work in the unfolding embryo, or rather the energies which the facts appear to imply.

Whatever natures we ascribe to the hypothetical units proper to each kind of organism, we must conclude that from the beginning of embryonic development, they have a proclivity towards the structure of that organism. Because of their phylogenetic origin, they must tend towards the form of the primitive type; but the superposed modifications, conflicting with their initial tendency, must cause a swerving towards each successively higher type. To take an illustration:--If in the germ-plasm out of which will come a vertebrate animal there is a proclivity towards the primitive piscine form, there must, if the germ-plasm is derived from a mammal, be also from the outset a proclivity towards the mammalian form. While the initial type tends continually to establish itself the terminal type tends also to establish itself. The intermediate structures must be influenced by their conflict, as well as by the conflict of each with the proclivities towards the amphibian and reptilian types. This complication of tendencies is increased by the intervention of several other factors.

There is the factor of economy. An embryo in which the transformations have absorbed the smallest amount of energy and wasted the smallest amount of matter, will have an advantage over embryos the transformations of which have cost more in energy and matter: the young animal will set out with a greater surplus of vitality, and will be more likely than others to live and propagate. Again, in the embryos of its descendants, inheriting the tendency to economical transformation, those which evolve at the least cost will thrive more than the rest and be more likely to have posterity. Thus will result a continual shortening of the processes. We can see alike that this must take place and that it does take place. If the whole series of phylogenetic changes had to be repeated--if the embryo mammal had to become a complete fish, and then a complete amphibian, and then a complete reptile, there would be an immense amount of superfluous building up and pulling down, entailing great waste of time and of materials. Evidently these abridgments which economy entails, necessitate that unfolding embryos bear but rude resemblances to lower types ancestrally passed through--vaguely represent their dominant traits only.

From this principle of economy arise several derivative principles, which may be best dealt with separately.

§ 130b. In some cases the substitution of an abridged for an unabridged course of evolution causes the entire disappearance of certain intermediate forms. Structural arrangements once passed through during the unfolding are dropped out of the series.

In the evolution of these embryos with which there is not laid up a large amount of food-yolk there occurs at the outset a striking omission of this kind. When, by successive fissions, the fertilized cell has given rise to a cluster of cells constituting a hollow sphere, known as a blastula, the next change under its original form is the introversion of one side, so as to produce two layers in place of one. An idea of the change may be obtained by taking an india-rubber ball (having a hole through which the air may escape) and thrusting in one side until its anterior surface touches the interior surface of the other side. If the cup-shaped structure resulting be supposed to have its wide opening gradually narrowed, until it becomes the mouth of an internal chamber, it will represent what is known as a gastrula--a double layer of cells, of which the outer is called epiblast and the inner hypoblast (answering to ectoderm and endoderm) inclosing a cavity known as the archenteron, or primitive digestive sac. But now in place of this original mode of forming the gastrula, there occurs a mode known as delamination. Throughout its whole extent the single layer splits so as to become a double layer--one sphere of cells inclosing the other; and after this direct formation of the double layer there is a direct formation of an opening through it into the internal cavity. There is thus a shortening of the primitive process: a number of changes are left out.

Often a kindred passing over of stages at later periods of development may be observed. In certain of the Mollusca, as the Patella chiton, the egg gives origin to a free-swimming larva known as a trochosphere, from which presently comes the ordinary molluscous organization. In the highest division of the Molluscs, however, the Cephalopods, no trochosphere is formed. The nutritive matter laid up in the egg is used in building up the young animal without any indication of an ancestral larva.

§ 130c. Among principles derived from the principle of economy is the principle of pre-adaptation--a name which we may appropriately coin to indicate an adaptation made in advance of the time at which it could have arisen in the course of phylogenetic history.

How pre-adaptation may result from economy will be shown by an illustration which human methods of construction furnish. Let us assume that building houses of a certain type has become an established habit, and that, as a part of each house, there is a staircase of given size. And suppose that in consequence of changed conditions--say the walling in of the town, limiting the internal space and increasing ground-rents--it becomes the policy to build houses of many stories, let out in flats to different tenants. For the increased passing up and down, a staircase wider at its lower part will be required. If now the builder, when putting up the ground floor, follows the old dimensions, then after all the stories are built, the lower part of the staircase, if it is to yield equal facilities for passage, must be reconstructed. Instead of a staircase adapted to those few stories which the original type of house had, economy will dictate a pre-adaptation of the staircase to the additional stories.

On carrying this idea with us, we shall see that if from some type of organism there is evolved a type in which enlargement of a certain part is needed to meet increased functions, the greater size of this part will begin to show itself during early stages of unfolding. That unbuilding and rebuilding which would be needful were it laid down of its original size, will be made needless if from the beginning it is laid down of a larger size. Hence, in successive generations, the greater prosperity and multiplication of individuals in which this part is at the outset somewhat larger than usual, must eventually establish a marked excess in its development at an early stage. The facts agree with this inference.

Referring to the contrasts between embryos, Mr. Adam Sedgwick says that "a species is distinct and distinguishable from its allies from the very earliest stages." Whereas, according to the law of von Baer, "animals so closely allied as the fowl and duck would be indistinguishable in the early stages of development," "yet I can distinguish a fowl and a duck embryo on the second day by the inspection of a single transverse section through the trunk." This experience harmonizes with the statement of the late Prof. Agassiz, that in some cases traits characterizing the species appear at an earlier period than traits characterizing the genus.

Similar in their implications are the facts recently published by Dr. E. Mehnert, concerning the feet of pentadactyle vertebrates. A leading example is furnished by the foot in the struthious birds. Out of the original five digits the two which eventually become large while the others disappear, soon give sign of their future predominance: their early sizes being in excess of those required for the usual functional requirements in birds, and preparing the way for their special requirements in the struthious birds. Dr. Mehnert shows that a like lesson is given by the relative developments of legs and wings in these birds. Ordinarily in vertebrates the fore limbs grow more rapidly than the hind limbs; but in the ostrich, in which the hind limbs or legs have to become so large while the wings are but little wanted, the leg development goes in advance of the wing-development in early embryonic stages: there is a pre-adaptation.

Much more striking are examples furnished by creatures whose modes of existence require that they shall have enormous fertility--require that the generative system shall be very large. Ordinarily the organs devoted to maintenance of the race develop later than the organs devoted to maintenance of the individual. But this order is inverted in certain Entozoa. To these creatures, imbedded in nutritive matters, self-maintenance cost nothing, and the structures devoted to it are relatively of less importance than the structures devoted to race-maintenance, which, to make up for the small chance any one germ has of getting into a fit habitat, have to produce immense numbers of germs. Here the rudiments of the generative systems are the first to become visible--here, in virtue of the principle of pre-adaptation, a structure belonging to the terminal form asserts itself so early in the developmental process as almost to obliterate the structure of the initial form.

It may be that in some cases where the growth of certain organs goes in advance of the normal order, the element of time comes into play--the greater time required for construction. To elucidate this let us revert to our simile. Suppose that the staircase above instanced, or at any rate its lower part, is required to be of marble with balusters finely carved. If this piece of work is not promptly commenced and pushed on fast, it will not be completed when the rest of the house is ready: workmen and tools will still block it up at a time when it should be available. Similarly among the parts of an unfolding embryo, those in which there is a great deal of constructive work must early take such shape as will allow of this. Now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. If this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems--why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish.

§ 130d. Yet another complication has to be noted. From the principle of economy, it seems inferable that decrease and disappearance of organs which were useful in ancestral types but have ceased to be useful, should take place uniformly; but they do not. In the words of Mr. Adam Sedgwick, "some ancestral organs persist in the embryo in a functionless rudimentary (vestigial) condition and at the same time without any reference to adult structures, while other ancestral organs have disappeared without leaving a trace."[46] This anomaly is rendered more striking when joined with the fact that some of the structures which remain conspicuous are relatively ancient, while some which have been obliterated are relatively modern--e. g., "gill slits [which date back to the fish-ancestor], have been retained in embryology, whereas other organs which have much more recently disappeared, e. g. teeth of birds, fore-limbs of snakes [dating back to the reptile ancestor], have been entirely lost."[47] Mr. Sedgwick ascribes these anomalies to the difference between larval development and embryonic development, and expresses his general belief thus:--

"The conclusion here reached is that, whereas larval development must retain traces (it may be very faint) of ancestral stages of structure because they are built out of ancestral stages, embryonic development need not necessarily do so, and very often does not; that embryonic development in so far as it is a record at all, is a record of structural features of previous larval stages. Characters which disappear during free life disappear also in the embryo, but characters which though lost by the adult are retained in the larva may ultimately be absorbed into the embryonic phase and leave their traces in embryonic development."[48]

To set forth the evidence justifying this view would encumber too much the general argument. Towards elucidation of such irregularities let me name two factors which should I think be taken into account.

Abridgment of embryonic stages cannot go on uniformly with all disused organs. Where an organ is of such size that progressive diminution of it will appreciably profit the young animal, by leaving it a larger surplus of unused material, we may expect progressive diminution to occur. Contrariwise, if the organ is relatively so small that each decrease will not, by sensibly increasing the reserve of nutriment, give the young animal an advantage over others, decrease must not be looked for: there may be a survival of it even though of very ancient origin.

Again, the reduction of a superfluous part can take place only on condition that the economy resulting from each descending variation of it, is of greater importance than are the effects of variations simultaneously occurring in other parts. If by increase or decrease of any other parts of the embryo, survival of the animal is furthered in a greater degree than by decrease of this superfluous part, then such decrease is unlikely; since it is illegitimate to count upon the repeated concurrence of favourable variations in two or more parts which are independent. So that if changes of an advantageous kind are going on elsewhere in the embryo a useless part may remain long undiminished.

Yet another cause operates, and perhaps cooperates. Embryonic survival of an organ which has become functionless, may readily happen if, during subsequent stages of development, parts of it are utilized as parts of other organs. In the words of Mr. J. T. Cunningham:--

"It seems to be a general fact that a structure which in metamorphosis disappears completely may easily be omitted altogether in embryonic development, while one which is modified into something else continues to pass more or less through its original larval condition." (Science Progress, July, 1897, p. 488.)

One more factor of considerable importance should be taken into account. A disused organ which entails evil because construction of it involves needless cost, may entail further evil by being in the way. This, it seems to me, is the reason why the fore-limbs of snakes have disappeared from their embryos. When the long-bodied lizard out of which the ophidian type evolved, crept through stiff herbage, and moved its head from side to side to find openings, there resulted alternate bends of its body, which were the beginnings of lateral undulations; and we may easily see that in proportion as it thus progressed by insinuating itself through interstices, the fore-limbs, less and less used for walking, would be more and more in the way; and the lengthening of the body, increasing the undulatory motion and decreasing the use of the fore-limbs, would eventually make them absolute impediments. Hence besides the benefit in economy of construction gained by embryos in which the fore-limbs were in early stages a little less developed than usual, they would gain an advantage by having, when mature, smaller fore-limbs than usual, leading to greater facility of locomotion. There would be a double set of influences causing, through selection, a comparatively rapid decrease of these appendages. And we may I think see also, on contemplating the kind of movement, that the fore-limbs would be more in the way than the hind limbs, which would consequently dwindle with such smaller rapidity as to make continuance of the rudiments of them comprehensible.

§ 131-132. So that while the embryonic law enunciated by von Baer is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies, the nonconformities to the law are also interpretable by this hypothesis.

Parallelism between the courses of development in species allied by remote ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, must obscure the embryonic history. And the principle of economy which leads to this substitution produces effects that are very irregular and uncertain in consequence of the endlessly varied conditions. Thus several causes conspire to produce deviations from the general law.

Let it be remarked, finally, that the ability to trace out embryologic kinships and the inability to do this, occur just where, according to the hypothesis of Evolution, they should occur. We saw in § 100a that zoologists are agreed in grouping animals into some 17 phyla--Mollusca, Arthropoda, Echinodermata, &c.--each of which includes a number of classes severally sub-divided into orders, genera, species. All the members of each phylum are so related embryologically, that the existence of a common ancestor of them in the remote past is considered certain. But when it comes to the relations among the archaic ancestors, opinion is unsettled. Whether, for instance, the primitive Chordata, out of which the Vertebrata emerged, have molluscan affinities or annelidan affinities, is still a matter in dispute. With regard to the origins of various other types no settled conclusions are held. Now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence.

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VI. THE ARGUMENTS FROM MORPHOLOGY

§ 133. Leaving out of consideration those parallelisms among their modes of development which characterize organisms belonging to each group, that community of plan which exists among them when mature is extremely remarkable and extremely suggestive. As before shown (§ 103), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations were practicable, nor the supposition of adherence to pre-determined typical plans, suffices to explain the facts. An instance will best prepare the reader for seeing the true meaning of these fundamental likenesses.

Under the immensely-varied forms of insects, greatly elongated like the dragon-fly or contracted in shape like the lady-bird, winged like the butterfly or wingless like the flea, we find this character in common--there are primarily seventeen segments.[49] These segments may be distinctly marked or they may be so fused as to make it difficult to find the divisions between them, but they always exist. What now can be the meaning of this community of structure throughout the hundred thousand kinds of insects filling the air, burrowing in the earth, swimming in the water? Why under the down-covered body of a moth and under the hard wing-cases of a beetle, should there be discovered the same number of divisions? Why should there be no more somites in the Stick-insect, or other Phasmid a foot long, than there are in a small creature like the louse? Why should the inert Aphis and the swift-flying Emperor-butterfly be constructed on the same fundamental plan? It cannot be by chance that there exist equal numbers of segments in all these multitudes of species. There is no reason to think it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design--to say that the Creator followed this pattern throughout, merely for the purpose of maintaining the pattern--is to assign an absurd motive. No rational interpretation of these and countless like morphological facts, can be given except by the hypothesis of evolution; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by perpetual divergences and re-divergences--if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fundamental structure among creatures which have severally become modified in multitudinous ways and degrees, in adaptation to their respective modes of life. To this let it be added that while the belief in an intentional adhesion to a pre-determined pattern throughout a whole group, is negatived by the occurrence of occasional deviations from the pattern; such deviations are reconcilable with the belief in evolution. As pointed out in the last chapter, ancestral traits will be obscured more or less according as the superposed modifications of structure, have or have not been furthered by the conditions of life and development to which the type has been subjected.

§ 134. Besides these wide-embracing and often deeply-hidden homologies, which hold together different animals, there are the scarcely-less significant homologies between different organs of the same animal. These, like the others, are obstacles to the supernatural interpretations and supports of the natural interpretation.

One of the most familiar and instructive examples is furnished by the vertebral column. Snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation. The movements would be impeded if, instead of a chain of vertebræ varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. But in the higher Vertebrata, the mechanical actions and reactions demand that while some parts of the vertebral column shall be flexible, other parts shall be inflexible. Inflexibility is specially requisite in that part of it called the sacrum; which, in mammals and birds, forms a fulcrum exposed to the greatest strains the skeleton has to bear. Now in both mammals and birds, this rigid portion of the vertebral column is not made of one long segment or vertebra, but of several segments fused together. In man there are five of these confluent sacral vertebræ; and in the ostrich tribe they number from seventeen to twenty. Why is this? Why, if the skeleton of each species was separately contrived, was this bony mass made by soldering together a number of vertebræ like those forming the rest of the column, instead of being made out of one single piece? And why, if typical uniformity was to be maintained, does the number of sacral vertebræ vary within the same order of birds? Why, too, should the development of the sacrum be by the round-about process of first forming its separate constituent vertebræ, and then destroying their separateness? In the embryo of a mammal or bird, the central element of the vertebral column is, at the outset, continuous. The segments that are to become vertebræ, arise gradually in the adjacent mesoderm, and enwrap this originally-homogeneous axis or notochord. Equally in those parts of the spine which are to remain flexible, and in those parts which are to grow rigid, these segments are formed; and that part of the spine which is to compose the sacrum, having acquired this segmental structure, loses it again by coalescence of the segments. To what end is this construction and re-construction? If, originally, the spine in vertebrate animals consisted from head to tail of separate moveable segments, as it does still in fishes and some reptiles--if, in the evolution of the higher Vertebrata, certain of these moveable segments were rendered less moveable with respect to one another, by the mechanical conditions they were exposed to, and at length became relatively immovable; it is comprehensible why the sacrum formed out of them, should continue ever after to show its originally-segmented structure. But on any other hypothesis this segmented structure is inexplicable. "We see the same law in comparing the wonderfully complex jaws and legs in crustaceans," says Mr. Darwin: referring to the fact that those numerous lateral appendages which, in the lower crustaceans, most of them serve as legs, and have like shapes, are, in the higher crustaceans, some of them represented by enormously-developed claws, and others by variously-modified foot-jaws. "It is familiar to almost every one," he continues, "that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spire. In monstrous plants we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike." ... "Why should one crustacean, which has an extremely complex mouth formed of many parts consequently always have fewer legs; or conversely, those with many legs have simpler mouths? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely-different purposes, be all constructed on the same pattern?"

To these and countless similar questions, the theory of evolution furnishes the only rational answer. In the course of that change from homogeneity to heterogeneity of structure displayed in evolution under every form, it will necessarily happen that from organisms made up of numerous like parts, there will arise organisms made up of parts more and more unlike: which unlike parts will nevertheless continue to bear traces of their primitive likeness.

§ 135. One more striking morphological fact, near akin to some of the facts dwelt on in the last chapter, must be here set down--the frequent occurrence, in adult animals and plants, of rudimentary and useless organs, which are homologous with organs that are developed and useful in allied animals and plants. In the last chapter we saw that during the development of embryos, there often arise organs which disappear on being replaced by other organs discharging the same functions in better ways; and that in some cases, organs develop to certain points and are then re-absorbed without performing any functions. Very generally, however, the partially-developed organs are retained throughout life.

The osteology of the higher Vertebrata supplies abundant examples. Vertebral processes which, in one tribe, are fully formed and ossified from independent centres, are, in other tribes, mere tubercles not having independent centres of ossification. While in the tail of this animal the vertebræ are severally composed of centrum and appendages, in the tail of that animal they are simple osseous masses without any appendages; and in another animal they have lost their individualities by coalescence with neighbouring vertebræ into a rudimentary tail. From the structures of the limbs analogous facts are cited by comparative anatomists. The undeveloped state of certain metacarpal bones, characterizes whole groups of mammals. In one case we find the normal number of digits; and, in another case, a smaller number with an atrophied digit to make out the complement. Here is a digit with its full number of phalanges; and there a digit of which one phalange has been arrested in its growth. Still more remarkable are the instances of entire limbs being rudimentary; as in certain snakes, which have hind legs hidden beneath the integument. So, too, is it with dermal appendages. Some of the smooth-skinned amphibia have scales buried in the skin. The seal, which is a mammal considerably modified in adaptation to an aquatic life, and which uses its feet mainly as paddles, has toes that still bear external nails; but the manatee, which is a much more transformed mammal, has nailless paddles which, when the skin is removed, are said, by Humboldt, to display rudimentary nails at the ends of the imbedded digits. Nearly all birds are covered with developed feathers, severally composed of a shaft bearing fibres, each of which, again, bears a fringe of down. But in some birds, as in the ostrich, various stages of arrested development of the feathers may be traced: between the unusually-elaborated feathers of the tail, and those about the beak which are reduced to simple hairs, there are transitions. Nor is this the extreme case. In the Apteryx we see the whole of the feathers reduced to a hair-like form. Again, the hair which commonly covers the body in mammals is, over the greater part of the human body almost rudimentary, and is in some parts reduced to mere down--down which nevertheless proves itself to be homologous with the hair of mammals in general, by occasionally developing into the original form. Numerous cases of aborted organs are given by Mr. Darwin, of which a few may be here added. "Nothing can be plainer," he remarks, "than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together?" ... "In plants with separated sexes, the male flowers often have a rudiment of a pistil; and Kölreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this shows that the rudiment and the perfect pistil are essentially alike in nature." And then, to complete the proof that these undeveloped parts are marks of descent from races in which they were developed, there are not a few direct experiences of this relation. "We have plenty of cases of rudimentary organs in our domestic productions--as the stump of a tail in tailless breeds--the vestige of an ear in earless breeds--the re-appearance of minute dangling horns in hornless breeds of cattle." (Origin of Species, 1859, pp. 451, 454.)

Here, as before, the teleological doctrine fails utterly; for these rudimentary organs are useless, and occasionally even detrimental; as is the appendix vermiformis, in Man--a part of the cæcum which is of no value for the purpose of absorption but which, by detaining small foreign bodies, often causes severe inflammation and death. The doctrine of typical plans is equally out of court; for while, in some members of a group, rudimentary organs completing the general type are traceable, in other members of the same group such organs are unrepresented. There remains only the doctrine of evolution; and to this, these rudimentary organs offer no difficulties. On the contrary, they are among its most striking evidences.

§ 136. The general truths of morphology thus coincide in their implications. Unity of type, maintained under extreme dissimilarities of form and mode of life, is explicable as resulting from descent with modification; but is otherwise inexplicable. The likenesses disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them; but they fit in quite harmoniously with the belief that each kind of organism is a product of accumulated modifications upon modifications. And the presence, in all kinds of animals and plants, of functionally-useless parts corresponding to parts that are functionally-useful in allied animals and plants, while it is totally incongruous with the belief in a construction of each organism by miraculous interposition, is just what we are led to expect by the belief that organisms have arisen by progression.

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X. ^A

XI. CLASSIFICATION

XII. DISTRIBUTION

I. PRELIMINARY

II. GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS.[45]

III. GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS

IV. THE ARGUMENTS FROM CLASSIFICATION

V. THE ARGUMENTS FROM EMBRYOLOGY

VI. THE ARGUMENTS FROM MORPHOLOGY